[Top] [Up] [Previous] [Next] [Index]

# 18 Cyclotomic Numbers

### Sections

1. Operations for Cyclotomics
2. Infinity
3. Comparisons of Cyclotomics
4. ATLAS Irrationalities
5. Galois Conjugacy of Cyclotomics
6. Internally Represented Cyclotomics

GAP admits computations in abelian extension fields of the rational number field Q, that is fields with abelian Galois group over Q. These fields are subfields of cyclotomic fields Q(en) where en = e2p i/n is a primitive complex n-th root of unity. The elements of these fields are called cyclotomics.

Information concerning operations for fields of cyclotomics, for example certain integral bases, can be found in Chapter Abelian Number Fields. For more general operations that take a field extension as a ---possibly optional--- argument, e.g., `Trace` or `Coefficients`, see Chapter Fields and Division Rings.

## 18.1 Operations for Cyclotomics

- `E(` n`) F`

  `E` returns the primitive n-th root of unity en = e2p i/n. Cyclotomics are usually entered as sums of roots of unity, with rational coefficients, and irrational cyclotomics are displayed in the same way. (For special cyclotomics, see ATLAS irrationalities.)

      gap> E(9); E(9)^3; E(6); E(12) / 3;
      -E(9)^4-E(9)^7
      E(3)
      -E(3)^2
      -1/3*E(12)^7

  A particular basis is used to express cyclotomics, see Integral Bases for Abelian Number Fields; note that `E(9)` is not a basis element, as the above example shows.

- `Cyclotomics V`

  is the field of all cyclotomics (in GAP).

      gap> E(9) in Cyclotomics; 37 in Cyclotomics; true in Cyclotomics;
      true
      true
      false

  As the cyclotomics are field elements the usual arithmetic operators `+`,`-`,`*` and `/` (and `^` to take powers by integers) are applicable. Note that `^` does not denote the conjugation of group elements, so it is not possible to explicitly construct groups of cyclotomics. (However, it is possible to compute the inverse and the multiplicative order of a nonzero cyclotomic.) Also, taking the k-th power of a cyclotomic is a Galois automorphism if and only if k is coprime to the conductor of the cyclotomic.

      gap> E(5) + E(3); (E(5) + E(5)^4) ^ 2; E(5) / E(3); E(5) * E(3);
      -E(15)^2-2*E(15)^8-E(15)^11-E(15)^13-E(15)^14
      -2*E(5)-E(5)^2-E(5)^3-2*E(5)^4
      E(15)^13
      E(15)^8
      gap> Order( E(5) ); Order( 1+E(5) );
      5
      infinity

- `IsCyclotomic(` obj`) C`
- `IsCyc(` obj`) C`

  Every object in the family `CyclotomicsFamily` lies in the category `IsCyclotomic`. This covers integers, rationals, proper cyclotomics, the object `infinity` (see Infinity), and unknowns (see Chapter Unknowns). All these objects except `infinity` and unknowns lie also in the category `IsCyc`, `infinity` lies in (and can be detected from) the category `IsInfinity`, and unknowns lie in `IsUnknown`.

      gap> IsCyclotomic(0); IsCyclotomic(1/2*E(3)); IsCyclotomic( infinity );
      true
      true
      true
      gap> IsCyc(0); IsCyc(1/2*E(3)); IsCyc( infinity );
      true
      true
      false

- `IsIntegralCyclotomic(` obj`) P`

  A cyclotomic is called integral or a cyclotomic integer if all coefficients of its minimal polynomial over the rationals are integers. Since the underlying basis of the external representation of cyclotomics is an integral basis (see Integral Bases for Abelian Number Fields), the subring of cyclotomic integers in a cyclotomic field is formed by those cyclotomics for which the external representation is a list of integers. For example, square roots of integers are cyclotomic integers (see ATLAS irrationalities), any root of unity is a cyclotomic integer, character values are always cyclotomic integers, but all rationals which are not integers are not cyclotomic integers.

      gap> r:= ER( 5 );                # The square root of 5 is a cyclotomic integer.
      E(5)-E(5)^2-E(5)^3+E(5)^4
      gap> IsIntegralCyclotomic( r );  # It has integral coefficients.
      true
      gap> r2:= 1/2 * r;               # This is not a cyclotomic integer, ...
      1/2*E(5)-1/2*E(5)^2-1/2*E(5)^3+1/2*E(5)^4
      gap> IsIntegralCyclotomic( r2 );
      false
      gap> r3:= 1/2 * r - 1/2;         # ... but this is one.
      E(5)+E(5)^4
      gap> IsIntegralCyclotomic( r3 );
      true

  The operation `Int` can be used to find a cyclotomic integer near to an arbitrary cyclotomic. For rationals, `Int` returns the largest integer smaller or equal to the argument.

      gap> Int( E(5)+1/2*E(5)^2 ); Int( 2/3*E(7)+3/2*E(4) );
      E(5)
      E(4)

  The operation `String` returns for a cyclotomic a string corresponding to the way the cyclotomic is printed by `ViewObj` and `PrintObj`.

      gap> String( E(5)+1/2*E(5)^2 ); String( 17/3 );
      "E(5)+1/2*E(5)^2"
      "17/3"

- `Conductor(` cyc`) A`
- `Conductor(` C`) A`

  For an element cyc of a cyclotomic field, `Conductor` returns the smallest integer n such that cyc is contained in the n-th cyclotomic field. For a collection C of cyclotomics (for example a dense list of cyclotomics or a field of cyclotomics), `Conductor` returns the smallest integer n such that all elements of C are contained in the n-th cyclotomic field.

      gap> Conductor( 0 ); Conductor( E(10) ); Conductor( E(12) );
      1
      5
      12

- `AbsoluteValue(` cyc`) A`

  returns the absolute value of a cyclotomic number cyc. At the moment only methods for rational numbers exist.

      gap> AbsoluteValue(-3);
      3

- `RoundCyc(` cyc`) O`

  is a cyclotomic integer z (see IsIntegralCyclotomic) near to the cyclotomic cyc in the sense that the i-th coefficient in the external representation (see CoeffsCyc) of z is `Int( c+1/2 )` where `c` is the i-th coefficient in the external representation of cyc. Expressed in terms of the Zumbroich basis (see Integral Bases for Abelian Number Fields), the coefficients of cyc w.r.t. this basis are rounded.

      gap> RoundCyc( E(5)+1/2*E(5)^2 ); RoundCyc( 2/3*E(7)+3/2*E(4) );
      E(5)+E(5)^2
      -2*E(28)^3+E(28)^4-2*E(28)^11-2*E(28)^15-2*E(28)^19-2*E(28)^23-2*E(28)^27

- `CoeffsCyc(` cyc`,` N`) F`

  Let cyc be a cyclotomic with conductor n. If N is not a multiple of n then `CoeffsCyc` returns `fail` because cyc cannot be expressed in terms of N-th roots of unity. Otherwise `CoeffsCyc` returns a list of length N with entry at position j equal to the coefficient of e2 p i (j-1)/N if this root belongs to the N-th Zumbroich basis (see Integral Bases for Abelian Number Fields), and equal to zero otherwise. So we have ``cyc`= CoeffsCyc(`cyc`,`N`) * List( [1..`N`], j -> E(`N`)^(j-1) )`.

      gap> cyc:= E(5)+E(5)^2;
      E(5)+E(5)^2
      gap> CoeffsCyc( cyc, 5 );  CoeffsCyc( cyc, 15 );  CoeffsCyc( cyc, 7 );
      [ 0, 1, 1, 0, 0 ]
      [ 0, -1, 0, 0, 0, 0, 0, 0, -1, 0, 0, -1, 0, -1, 0 ]
      fail

- `DenominatorCyc(` cyc`) F`

  For a cyclotomic number cyc (see IsCyclotomic), this function returns the smallest positive integer n such that ``n`*` cyc`` is a cyclotomic integer (see IsIntegralCyclotomic). For rational numbers cyc, the result is the same as that of `DenominatorRat` (see DenominatorRat).

- `ExtRepOfObj(`cyc`)`

      gap> ExtRepOfObj( E(5) ); CoeffsCyc( E(5), 15 );
      [ 0, 1, 0, 0, 0 ]
      [ 0, 0, 0, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, -1, 0 ]
      gap> CoeffsCyc( 1+E(3), 9 ); CoeffsCyc( E(5), 7 );
      [ 0, 0, 0, 0, 0, 0, -1, 0, 0 ]
      fail

- `DescriptionOfRootOfUnity(` root`) F`

  Given a cyclotomic root that is known to be a root of unity (this is not checked), `DescriptionOfRootOfUnity` returns a list [ n, e ] of coprime positive integers such that root = `E`(n)e holds.

      gap> E(9);  DescriptionOfRootOfUnity( E(9) );
      -E(9)^4-E(9)^7
      [ 9, 1 ]
      gap> DescriptionOfRootOfUnity( -E(3) );
      [ 6, 5 ]

- `IsGaussInt(` x`) F`

  `IsGaussInt` returns `true` if the object x is a Gaussian integer and `false` otherwise. Gaussian integers are of the form ``a`+` b`*E(4)`, where a and b are integers.

- `IsGaussRat(` x`) F`

  `IsGaussRat` returns `true` if the object x is a Gaussian rational and `false` otherwise. Gaussian rationals are of the form ``a`+` b`*E(4)`, where a and b are rationals.

  `DefaultField` (see DefaultField) for cyclotomics is defined to return the smallest cyclotomic field containing the given elements.

      gap> Field( E(5)+E(5)^4 );  DefaultField( E(5)+E(5)^4 );
      NF(5,[ 1, 4 ])
      CF(5)

  ## 18.2 Infinity

- `IsInfinity(` obj`) C`
- `infinity V`

  `infinity` is a special GAP object that lies in `CyclotomicsFamily`. It is larger than all other objects in this family. `infinity` is mainly used as return value of operations such as `Size` and `Dimension` for infinite resp. infinite dimensional domains.

  Note that no arithmetic operations are provided for `infinity`, in particular there is no problem to define what `0 * infinity` or `infinity - infinity` means.

  Often it is useful to distinguish `infinity` from ``proper'' cyclotomics. For that, `infinity` lies in the category `IsInfinity` but not in `IsCyc`, and the other cyclotomics lie in the category `IsCyc` but not in `IsInfinity`.

      gap> s:= Size( Rationals );
      infinity
      gap> s = infinity; IsCyclotomic( s ); IsCyc( s ); IsInfinity( s );
      true
      true
      false
      true
      gap> s in Rationals; s > 17;
      false
      true
      gap> Set( [ s, 2, s, E(17), s, 19 ] );
      [ 2, 19, E(17), infinity ]

  ## 18.3 Comparisons of Cyclotomics

  To compare cyclotomics, the operators `<`, `<=`, `=`, `>=`, `>` and `<>` can be used, the result will be `true` if the first operand is smaller, smaller or equal, equal, larger or equal, larger, or unequal, respectively, and `false` otherwise.

  Cyclotomics are ordered as follows: The relation between rationals is the natural one, rationals are smaller than irrational cyclotomics, and `infinity` is the largest cyclotomic. For two irrational cyclotomics with different conductors, the one with smaller conductor is regarded as smaller. Two irrational cyclotomics with same conductor are compared via their external representation.

  For comparisons of cyclotomics and other GAP objects, see Section Comparisons.

      gap> E(5) < E(6);      # the latter value has conductor 3
      false
      gap> E(3) < E(3)^2;    # both have conductor 3, compare the ext. repr.
      false
      gap> 3 < E(3); E(5) < E(7);
      true
      true

  ## 18.4 ATLAS Irrationalities

- `EB(` n`) F`
- `EC(` n`) F`
- `ED(` n`) F`
- `EE(` n`) F`
- `EF(` n`) F`
- `EG(` n`) F`
- `EH(` n`) F`

  For N a positive integer, let z = `E``(`N `)` = exp(2 p i/N). The following so-called atomic irrationalities (see Chapter 7, Section 10 of CCN85) can be entered using functions. (Note that the values are not necessary irrational.)

  `EB``(`N `)`
  =
  bN
  =
  | 1 2 |  | N-1 å j=1 | zj2,
  N º 1 mod 2
  `EC``(`N `)`
  =
  cN
  =
  | 1 3 |  | N-1 å j=1 | zj3,
  N º 1 mod 3
  `ED``(`N `)`
  =
  dN
  =
  | 1 4 |  | N-1 å j=1 | zj4,
  N º 1 mod 4
  `EE``(`N `)`
  =
  eN
  =
  | 1 5 |  | N-1 å j=1 | zj5,
  N º 1 mod 5
  `EF``(`N `)`
  =
  fN
  =
  | 1 6 |  | N-1 å j=1 | zj6,
  N º 1 mod 6
  `EG``(`N `)`
  =
  gN
  =
  | 1 7 |  | N-1 å j=1 | zj7,
  N º 1 mod 7
  `EH``(`N `)`
  =
  hN
  =
  | 1 8 |  | N-1 å j=1 | zj8,
  N º 1 mod 8

  (Note that in cN, ¼, hN, N must be a prime.)

- `EI(` n`) F`
- `ER(` n`) F`

  For a rational number N, `ER` returns the square root Ö{N } of N, and `EI` returns Ö{-N }. By the chosen embedding of cyclotomic fields into the complex numbers, `ER` returns the positive square root if N is positive, and if N is negative then `ER(`N`) = EI(-`N`)`. In any case, `EI(`N`) = E(4) * ER(`N`)`.

  `ER` is installed as method for the operation `Sqrt` (see Sqrt) for rational argument.

  From a theorem of Gauss we know that

  bN = | ì ï ï ï ï í ï ï ï ï î |  |
  | 1 2 | (-1+Ö N)
  if
  N º 1
  mod 4
  | 1 2 | (-1+i Ö N)
  if
  N º -1
  mod 4

  So Ö N can be computed from bN (see EB).

- `EY(` n`[,` d`] ) F`
- `EX(` n`[,` d`] ) F`
- `EW(` n`[,` d`] ) F`
- `EV(` n`[,` d`] ) F`
- `EU(` n`[,` d`] ) F`
- `ET(` n`[,` d`] ) F`
- `ES(` n`[,` d`] ) F`

  For given N, let nk = nk(N) be the first integer with multiplicative order exactly k modulo N, chosen in the order of preference

  1, -1, 2, -2, 3, -3, 4, -4, ¼.

  We define

  `EY``(`N `)`
  =
  yn
  =
  z+zn
  (n=n2)
  `EX``(`N `)`
  =
  xn
  =
  z+zn+zn2
  (n=n3)
  `EW``(`N `)`
  =
  wn
  =
  z+zn+zn2+zn3
  (n=n4)
  `EV``(`N `)`
  =
  vn
  =
  z+zn+zn2+zn3+zn4
  (n=n5)
  `EU``(`N `)`
  =
  un
  =
  z+zn+zn2+ ¼+zn5
  (n=n6)
  `ET``(`N `)`
  =
  tn
  =
  z+zn+zn2+ ¼+zn6
  (n=n7)
  `ES``(`N `)`
  =
  sn
  =
  z+zn+zn2+ ¼+zn7
  (n=n8)

- `EM(` n`[,` d`] ) F`
- `EL(` n`[,` d`] ) F`
- `EK(` n`[,` d`] ) F`
- `EJ(` n`[,` d`] ) F`

  `EM``(`N `)`
  =
  mn
  =
  z-zn
  (n=n2)
  `EL``(`N `)`
  =
  ln
  =
  z-zn+zn2-zn3
  (n=n4)
  `EK``(`N `)`
  =
  kn
  =
  z-zn+ ¼-zn5
  (n=n6)
  `EJ``(`N `)`
  =
  jn
  =
  z-zn+ ¼-zn7
  (n=n8)

- `NK(` n`,` k`,` d`) F`

  Let nk(d) = nk(d)(N) be the d+1-th integer with multiplicative order exactly k modulo N, chosen in the order of preference defined above; we write nk=nk(0),nk¢=nk(1), nk¢¢ = nk(2) and so on. These values can be computed as `NK(`N`,`k`,`d`)` = nk(d)(N); if there is no integer with the required multiplicative order, `NK` returns `fail`.

  The algebraic numbers

  yN¢=yN(1),yN¢¢=yN(2),¼, xN¢,xN¢¢,¼,jN¢,jN¢¢,¼

  are obtained on replacing nk in the above definitions by nk¢,nk¢¢,¼; they can be entered as

  `EY``(`N `,`d `)`
  =
  yN(d)
  `EX``(`N `,`d `)`
  =
  xN(d)
  :
  `EJ``(`N `,`d `)`
  =
  jn(d)

- `AtlasIrrationality(` irratname`) F`

  Let irratname be a string that describes an irrational value as described in Chapter 6, Section 10 of CCN85, that is, a linear combination of the atomic irrationalities introduced above. (The following definition is mainly copied from CCN85.) If qN is such a value (e.g. y24¢¢) then linear combinations of algebraic conjugates of qN are abbreviated as in the following examples:

  `2` `qN``+``3``&``5`-`4``&``7``+``&``9`
  means
  2 qN + 3 qN*5 - 4 qN*7 + qN*9
  `4` `qN``&``3``&``5``&``7`-`3``&``4`
  means
  4 (qN + qN*3 + qN*5 + qN*7) - 3 qN*11
  `4` `qN``*``3``&``5``+``&``7`
  means
  4 (qN*3 + qN*5) + qN*7

  To explain the ``ampersand'' syntax in general we remark that ``&k'' is interpreted as qN*k, where qN is the most recently named atomic irrationality, and that the scope of any premultiplying coefficient is broken by a + or - sign, but not by & or *k. The algebraic conjugations indicated by the ampersands apply directly to the atomic irrationality qN, even when, as in the last example, qN first appears with another conjugacy *k.

      gap> EW(16,3); EW(17,2); ER(3); EI(3); EY(5); EB(9);
      0
      E(17)+E(17)^4+E(17)^13+E(17)^16
      -E(12)^7+E(12)^11
      E(3)-E(3)^2
      E(5)+E(5)^4
      1
      gap> AtlasIrrationality( "b7*3" );
      E(7)^3+E(7)^5+E(7)^6
      gap> AtlasIrrationality( "y'''24" );
      E(24)-E(24)^19
      gap> AtlasIrrationality( "-3y'''24*13&5" );
      3*E(8)-3*E(8)^3
      gap> AtlasIrrationality( "3y'''24*13-2&5" );
      -3*E(24)-2*E(24)^11+2*E(24)^17+3*E(24)^19
      gap> AtlasIrrationality( "3y'''24*13-&5" );
      -3*E(24)-E(24)^11+E(24)^17+3*E(24)^19
      gap> AtlasIrrationality( "3y'''24*13-4&5&7" );
      -7*E(24)-4*E(24)^11+4*E(24)^17+7*E(24)^19
      gap> AtlasIrrationality( "3y'''24&7" );
      6*E(24)-6*E(24)^19

  ## 18.5 Galois Conjugacy of Cyclotomics

- `GaloisCyc(` cyc`,` k`) O`
- `GaloisCyc(` list`,` k`) O`

  For a cyclotomic cyc and an integer k, `GaloisCyc` returns the cyclotomic obtained by raising the roots of unity in the Zumbroich basis representation of cyc to the k-th power. If k is coprime to the integer n, `GaloisCyc( .,` k`)` acts as a Galois automorphism of the n-th cyclotomic field (see Galois Groups of Abelian Number Fields); to get the Galois automorphisms themselves, use `GaloisGroup` (see GaloisGroup!of field).

  The complex conjugate of cyc is `GaloisCyc(` cyc`, -1 )`, which can also be computed using `ComplexConjugate` (see ComplexConjugate).

  For a list or matrix list of cyclotomics, `GaloisCyc` returns the list obtained by applying `GaloisCyc` to the entries of list.

      gap> GaloisCyc( E(5) + E(5)^4, 2 );
      E(5)^2+E(5)^3
      gap> GaloisCyc( E(5), -1 );           # the complex conjugate
      E(5)^4
      gap> GaloisCyc( E(5) + E(5)^4, -1 );  # this value is real
      E(5)+E(5)^4
      gap> GaloisCyc( E(15) + E(15)^4, 3 );
      E(5)+E(5)^4
      gap> ComplexConjugate( E(7) );
      E(7)^6

- `StarCyc(` cyc`) F`

  If the cyclotomic cyc is an irrational element of a quadratic extension of the rationals then `StarCyc` returns the unique Galois conjugate of cyc that is different from cyc, otherwise `fail` is returned. In the first case, the return value is often called cyc * (see Printing Character Tables).

      gap> StarCyc( EB(5) ); StarCyc( E(5) );
      E(5)^2+E(5)^3
      fail

- `Quadratic(` cyc`) F`

  Let cyc be a cyclotomic integer that lies in a quadratic extension field of the rationals. Then we have cyc = (a + b Ö n) / d for integers a, b, n, d, such that d is either 1 or 2. In this case, `Quadratic` returns a record with the components `a`, `b`, `root`, `d`, `ATLAS`, and `display`; the values of the first four are a, b, n, and d, the `ATLAS` value is a (not necessarily shortest) representation of cyc in terms of the ATLAS irrationalities b|n|, i|n|, r|n|, and the `display` value is a string that expresses cyc in GAP notation, corresponding to the value of the `ATLAS` component.

  If cyc is not a cyclotomic integer or does not lie in a quadratic extension field of the rationals then `fail` is returned.

  If the denominator d is 2 then necessarily n is congruent to 1 modulo 4, and rn, in are not possible; we have ``cyc`= x + y * EB( root )` with `y = b`, `x = ( a + b ) / 2`.

  If d = 1, we have the possibilities i|n| for n < -1, a + b * i for n = -1, a + b * rn for n > 0. Furthermore if n is congruent to 1 modulo 4, also cyc = (a+b) + 2 * b * b|n| is possible; the shortest string of these is taken as the value for the component `ATLAS`.

      gap> Quadratic( EB(5) ); Quadratic( EB(27) );
      rec( a := -1, b := 1, root := 5, d := 2, ATLAS := "b5",
        display := "(-1+ER(5))/2" )
      rec( a := -1, b := 3, root := -3, d := 2, ATLAS := "1+3b3",
        display := "(-1+3*ER(-3))/2" )
      gap> Quadratic(0); Quadratic( E(5) );
      rec( a := 0, b := 0, root := 1, d := 1, ATLAS := "0", display := "0" )
      fail

- `GaloisMat(` mat`) A`

  Let mat be a matrix of cyclotomics. `GaloisMat` calculates the complete orbits under the operation of the Galois group of the (irrational) entries of mat, and the permutations of rows corresponding to the generators of the Galois group.

  If some rows of mat are identical, only the first one is considered for the permutations, and a warning will be printed.

  `GaloisMat` returns a record with the components `mat`, `galoisfams`, and `generators`.

  `mat`:
    a list with initial segment being the rows of mat (not shallow copies of these rows); the list consists of full orbits under the action of the Galois group of the entries of mat defined above. The last rows in the list are those not contained in mat but must be added in order to complete the orbits; so if the orbits were already complete, mat and `mat` have identical rows.

  `galoisfams`:
    a list that has the same length as the `mat` component, its entries are either 1, 0, -1, or lists. `galoisfams[i] = 1` means that `mat[i]` consists of rationals, i.e. `[ mat[i] ]` forms an orbit; `galoisfams[i] = -1` means that `mat[i]` contains unknowns (see Chapter Unknowns); in this case `[ mat[i] ]` is regarded as an orbit, too, even if `mat[i]` contains irrational entries; if `galoisfams``[``i``]` = [ l1, l2 ] is a list then `mat[i]` is the first element of its orbit in `mat`, l1 is the list of positions of rows that form the orbit, and l2 is the list of corresponding Galois automorphisms (as exponents, not as functions), so we have `mat`[ l1[j] ][k] = `GaloisCyc`( `mat`[i][k], l2[j] ); `galoisfams[i] = 0` means that `mat[i]` is an element of a nontrivial orbit but not the first element of it.

  `generators`:
    a list of permutations generating the permutation group corresponding to the action of the Galois group on the rows of `mat`.

  In the following example we temporarily increase the line length limit from its default value 80 to 84 in order to get a nicer output format.

      gap> SizeScreen([ 84, ]);;
      gap> GaloisMat( [ [ E(3), E(4) ] ] );
      rec(
        mat := [ [ E(3), E(4) ], [ E(3), -E(4) ], [ E(3)^2, E(4) ], [ E(3)^2, -E(4) ] ],
        galoisfams := [ [ [ 1, 2, 3, 4 ], [ 1, 7, 5, 11 ] ], 0, 0, 0 ],
        generators := [ (1,2)(3,4), (1,3)(2,4) ] )
      gap> SizeScreen([ 80, ]);;
      gap> GaloisMat( [ [ 1, 1, 1 ], [ 1, E(3), E(3)^2 ] ] );
      rec( mat := [ [ 1, 1, 1 ], [ 1, E(3), E(3)^2 ], [ 1, E(3)^2, E(3) ] ],
        galoisfams := [ 1, [ [ 2, 3 ], [ 1, 2 ] ], 0 ], generators := [ (2,3) ] )

- `RationalizedMat(` mat`) A`

  returns the list of rationalized rows of mat, which must be a matrix of cyclotomics. This is the set of sums over orbits under the action of the Galois group of the entries of mat (see GaloisMat), so the operation may be viewed as a kind of trace on the rows.

  Note that no two rows of mat should be equal.

      gap> mat:=List(Irr(CharacterTable( "A5" )),ValuesOfClassFunction);
      [ [ 1, 1, 1, 1, 1 ], [ 3, -1, 0, -E(5)-E(5)^4, -E(5)^2-E(5)^3 ],
        [ 3, -1, 0, -E(5)^2-E(5)^3, -E(5)-E(5)^4 ], [ 4, 0, 1, -1, -1 ],
        [ 5, 1, -1, 0, 0 ] ]
      gap> RationalizedMat( mat );
      [ [ 1, 1, 1, 1, 1 ], [ 6, -2, 0, 1, 1 ], [ 4, 0, 1, -1, -1 ],
        [ 5, 1, -1, 0, 0 ] ]

  ## 18.6 Internally Represented Cyclotomics

  The implementation of an internally represented cyclotomic is based on a list of length equal to its conductor. This means that the internal representation of a cyclotomic does not refer to the smallest number field but the smallest cyclotomic field containing it. The reason for this is the wish to reflect the natural embedding of two cyclotomic fields into a larger one that contains both. With such embeddings, it is easy to construct the sum or the product of two arbitrary cyclotomics as an element of a cyclotomic field.

  The disadvantage of this approach is that the arithmetical operations are quite expensive, so the use of internally represented cyclotomics is not recommended for doing arithmetics over number fields, such as calculations with matrices of cyclotomics. But internally represented cyclotomics are good enough for dealing with irrationalities in character tables (see chapter Character Tables).

  For the representation of cyclotomics one has to recall that the n-th cyclotomic field Q(en) is a vector space of dimension j(n) over the rationals where j denotes Euler's phi-function (see Phi).

  A special integral basis of cyclotomic fields is chosen that allows one to easily convert arbitrary sums of roots of unity into the basis, as well as to convert a cyclotomic represented w.r.t. the basis into the smallest possible cyclotomic field. This basis is accessible in GAP, see Integral Bases for Abelian Number Fields for more information and references.

  Note that the set of all n-th roots of unity is linearly dependent for n > 1, so multiplication is not the multiplication of the group ring Q á en ñ; given a Q-basis of Q(en) the result of the multiplication (computed as multiplication of polynomials in en, using (en)n = 1) will be converted to the basis.

      gap> E(5) * E(5)^2; ( E(5) + E(5)^4 ) * E(5)^2;
      E(5)^3
      E(5)+E(5)^3
      gap> ( E(5) + E(5)^4 ) * E(5);
      -E(5)-E(5)^3-E(5)^4

  An internally represented cyclotomic is always represented in the smallest cyclotomic field it is contained in. The internal coefficients list coincides with the external representation returned by `ExtRepOfObj`.

  Since the conductor of internally represented cyclotomics must be in the category `IsSmallIntRep`, the biggest possible (though not very useful) conductor is 65535. So the maximal cyclotomic field implemented in GAP is not really the field Qab.

  [Top] [Up] [Previous] [Next] [Index]

  GAP 4 manual
  May 2002