# 13 Cyclotomics

GAP allows computations in abelian extension fields of the rational field Q, i.e., fields with abelian Galois group over Q. These fields are described in chapter Subfields of Cyclotomic Fields. They are subfields of cyclotomic fields Q_n = Q(e_n) where e_n = e^{frac{2pi i}{n}} is a primitive n--th root of unity. Their elements are called cyclotomics.

The internal representation of a cyclotomic does not refer to the smallest number field but the smallest cyclotomic field containing it (the so--called conductor). This is because it is easy to embed two cyclotomic fields in a larger one that contains both, i.e., there is a natural way to get the sum or the product of two arbitrary cyclotomics as element of a cyclotomic field. The disadvantage is that the arithmetical operations are too expensive to do arithmetics in number fields, e.g., calculations in a matrix ring over a number field. But it suffices to deal with irrationalities in character tables (see Character Tables). (And in fact, the comfortability of working with the natural embeddings is used there in many situations which did not actually afford it ldots)

All functions that take a field extension as ---possibly optional--- argument, e.g., Trace or Coefficients (see chapter Fields), are described in chapter Subfields of Cyclotomic Fields.

the representation of cyclotomics in GAP (see More about Cyclotomics),
integral elements of number fields (see Cyclotomic Integers, IntCyc, RoundCyc),
characteristic functions (see IsCyc, IsCycInt),
comparison and arithmetical operations of cyclotomics (see Comparisons of Cyclotomics, Operations for Cyclotomics),
functions concerning Galois conjugacy of cyclotomics (see GaloisCyc, StarCyc), or lists of them (see GaloisMat, RationalizedMat),
some special cyclotomics, as defined in CCN85 (see ATLAS irrationalities, Quadratic)

The external functions are in the file LIBNAME/"cyclotom.g".

### Subsections

Subfields of Cyclotomic Fields), cyclotomics for short, are arithmetical objects like rationals and finite field elements; they are not implemented as records ---like groups--- or e.g. with respect to a character table (although character tables may be the main interest for cyclotomic arithmetics).

E( n )

returns the primitive n-th root of unity e_n = e^{frac{2pi i}{n}}. Cyclotomics are usually entered as (and irrational cyclotomics are always displayed as) sums of roots of unity with ATLAS irrationalities.)

gap> E(9); E(9)^3; E(6); E(12) / 3;
-E(9)^4-E(9)^7    # the root needs not to be an element of the base
E(3)
-E(3)^2
-1/3*E(12)^7

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

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 < e_n > ; given a Q-basis of Q_n the result of the multiplication (computed as multiplication of polynomials in e_n, using (e_n)^n = 1) will be converted to the base.

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

Cyclotomics are always represented in the smallest cyclotomic field they are contained in. Together with the choice of a fixed base this means that two cyclotomics are equal if and only if they are equally represented.

Addition and multiplication of two cyclotomics represented in Q_n and Q_m, respectively, is computed in the smallest cyclotomic field containing both: Q_{'Lcm'(n,m)}. Conversely, if the result is contained in a smaller cyclotomic field the representation is reduced to the minimal such field.

The base, the base conversion and the reduction to the minimal cyclotomic field are described in~Zum89, more about the base can be found in ZumbroichBase.

Since n must be a short integer, the maximal cyclotomic field implemented in GAP is not really the field Q^{ab}. The biggest allowed (though not very useful) n is 65535.

There is a global variable Cyclotomics in GAP, a record that Subfields of Cyclotomic Fields).

## 13.2 Cyclotomic Integers

A cyclotomic is called integral or cyclotomic integer if all coefficients of its minimal polynomial are integers. Since the base used is an integral base (see ZumbroichBase), the subring of cyclotomic integers in a cyclotomic field is formed by those cyclotomics which have not only rational but integral coefficients in their representation as sums of roots of unity. For example, square 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. (See IsCycInt)

gap> ER( 5 );                # The square root of 5 is a cyclotomic
E(5)-E(5)^2-E(5)^3+E(5)^4    # integer, it has integral coefficients.
gap> 1/2 * ER( 5 );          # This is not a cyclotomic integer, \ldots
1/2*E(5)-1/2*E(5)^2-1/2*E(5)^3+1/2*E(5)^4
gap> 1/2 * ER( 5 ) - 1/2;    # \ldots but this is one.
E(5)+E(5)^4

## 13.3 IntCyc

IntCyc( z )

returns the cyclotomic integer (see Cyclotomic Integers) with Zumbroich base coefficients (see ZumbroichBase) List( zumb, x - Int( x ) ) where zumb is the vector of Zumbroich base coefficients of the cyclotomic z; see also RoundCyc.

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

## 13.4 RoundCyc

RoundCyc( z )

returns the cyclotomic integer (see Cyclotomic Integers) with Zumbroich base coefficients (see ZumbroichBase) List( zumb, x - Int( x+1/2 ) ) where zumb is the vector of Zumbroich base coefficients of the cyclotomic z; see also IntCyc.

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

## 13.5 IsCyc

IsCyc( obj )

returns true if obj is a cyclotomic, and false otherwise. Will signal an error if obj is an unbound variable.

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

IsCyc is an internal function.

## 13.6 IsCycInt

IsCycInt( obj )

Cyclotomic Integers), false otherwise. Will signal an error if obj is an unbound variable.

gap> IsCycInt( 0 ); IsCycInt( E(3) ); IsCycInt( 1/2 * E(3) );
true
true
false

IsCycInt is an internal function.

## 13.7 NofCyc

NofCyc( z )
NofCyc( list )

returns the smallest positive integer n for which the cyclotomic z is resp. for which all cyclotomics in the list list are contained in Q_n = Q( e^{frac{2 pi i}{n}} ) = Q( 'E(<n>)' ).

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

NofCyc is an internal function.

## 13.8 CoeffsCyc

CoeffsCyc( z, n )

If z is a cyclotomic which is contained in Q_n, CoeffsCyc( z, n ) returns a list cfs of length n where the entry at position i is the coefficient of 'E(<n>)'^{i-1} in the internal representation of z as element of the cyclotomic field Q_n (see More about Cyclotomics, ZumbroichBase): <z> = <cfs>[1] + <cfs>[2] 'E(<n>)'^1 + ldots + <cfs>[n] 'E(<n>)'^{n-1}.

Note that all positions which do not belong to base elements of Q_n contain zeroes.

gap> CoeffsCyc( E(5), 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 ]
Error, no representation of <z> in 7th roots of unity

CoeffsCyc calls the internal function COEFFSCYC:

COEFFSCYC( z )

is equivalent to CoeffsCyc( z, NofCyc( z ) ), see NofCyc.

## 13.9 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 inequal, respectively, and false otherwise.

Cyclotomics are ordered as follows: The relation between rationals is as usual, and rationals are smaller than irrational cyclotomics. For two irrational cyclotomics z1, z2 which lie in different minimal cyclotomic fields, we have <z1> < <z2> if and only if 'NofCyc(<z1>)' < 'NofCyc(<z2>)'); if 'NofCyc(<z1>)' = 'NofCyc(<z2>)'), that one is smaller that has the smaller coefficient vector, i.e., <z1> leq <z2> if and only if 'COEFFSCYC(<z1>)' leq 'COEFFSCYC(<z2>)'.

You can compare cyclotomics with objects of other types; all objects which are not cyclotomics are larger than cyclotomics.

gap> E(5) < E(6);     # the latter value lies in \$Q_3\$
false
gap> E(3) < E(3)^2;    # both lie in \$Q_3\$, so compare coefficients
false
gap> 3 < E(3); E(5) < E(7);
true
true
gap> E(728) < (1,2);
true

## 13.10 Operations for Cyclotomics

The operators +, -, *, / are used for addition, subtraction, multiplication and division of two cyclotomics; note that division by 0 causes an error.

+ and - can also be used as unary operators;

^ is used for exponentiation of a cyclotomic with an integer; this is in general not equal to Galois conjugation.

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

## 13.11 GaloisCyc

GaloisCyc( z, k )

returns the cyclotomic obtained on raising the roots of unity in the representation of the cyclotomic z to the k-th power. If z is represented in the field Q_n and k is a fixed integer relative prime to n, GaloisCyc( ., k ) acts as a Galois automorphism of Q_n (see GaloisGroup for Number Fields); to get Galois automorphisms as functions, use GaloisGroup GaloisGroup.

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

GaloisCyc is an internal function.

## 13.12 ATLAS irrationalities

EB( N ), EC( N ), ldots, EH( N ),
EI( N ), ER( N ),
EJ( N ), EK( N ), EL( N ), EM( N ),
EJ( N, d ), EK( N, d ), EL( N, d ), EM( N, d ),
ES( N ), ET( N ), ldots, EY( N ),
ES( N, d ), ET( N, d ), ldots, EY( N, d ),
NK( N, k, d )

For N a positive integer, let z = 'E(<N>)' = e^{2 pi i / N}. The following so-called atomic irrationalities (see~cite[Chapter 7, Section 10]CCN85) can be entered by functions (Note that the values are not necessary irrational.):

[beginarrayllllll EB(N) & = & b_N & = & frac12sum_j=1^N-1z^j^2 & (Nequiv 1bmod 2)

EC(N) & = & c_N & = & frac13sum_j=1^N-1z^j^3 & (Nequiv 1bmod 3)

ED(N) & = & d_N & = & frac14sum_j=1^N-1z^j^4 & (Nequiv 1bmod 4)

EE(N) & = & e_N & = & frac15sum_j=1^N-1z^j^5 & (Nequiv 1bmod 5)

EF(N) & = & f_N & = & frac16sum_j=1^N-1z^j^6 & (Nequiv 1bmod 6)

EG(N) & = & g_N & = & frac17sum_j=1^N-1z^j^7 & (Nequiv 1bmod 7)

EH(N) & = & h_N & = & frac18sum_j=1^N-1z^j^8 & (Nequiv 1bmod 8)

(Note that in c_N, ldots, h_N, N must be a prime.)

[beginarraylllll ER(N) & = & sqrtN
EI(N) & = & i sqrtN & = & sqrt-N

From a theorem of Gauss we know that [ b_N = left{ beginarrayllll frac12(-1+sqrtN) & rm if & Nequiv 1 & bmod 4
frac12(-1+isqrtN) & rm if & Nequiv -1 & bmod 4 endarrayright. ,]

so sqrt{N} can be (and in fact is) computed from b_N. If N is a negative integer then ER(N) = EI(-N).

For given N, let n_k = n_k(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, ldots .]

We have [beginarrayllllll EY(N) & = & y_n & = & z+z^n &(n = n_2)
EX(N) & = & x_n & = & z+z^n+z^n^2 &(n=n_3)
EW(N) & = & w_n & = & z+z^n+z^n^2+z^n^3 &(n=n_4)
EV(N) & = & v_n & = & z+z^n+z^n^2+z^n^3+z^n^4 &(n=n_5)
EU(N) & = & u_n & = & z+z^n+z^n^2+ldots +z^n^5 &(n=n_6)
ET(N) & = & t_n & = & z+z^n+z^n^2+ldots +z^n^6 &(n=n_7)
ES(N) & = & s_n & = & z+z^n+z^n^2+ldots +z^n^7 &(n=n_8)

[beginarrayllllll EM(N) & = & m_n & = & z-z^n &(n=n_2)
EL(N) & = & l_n & = & z-z^n+z^n^2-z^n^3 &(n=n_4)
EK(N) & = & k_n & = & z-z^n+ldots -z^n^5 &(n=n_6)
EJ(N) & = & j_n & = & z-z^n+ldots -z^n^7 &(n=n_8)

Let n_k^{(d)} = n_k^{(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 n_k=n_k^{(0)},n_k^{prime}=n_k^{(1)}, n_k^{primeprime} = n_k^{(2)} and so on. These values can be computed as NK(N,k,d) = n_k^{(d)}(N); if there is no integer with the required multiplicative order, NK will return false.

The algebraic numbers [y_N^prime=y_N^(1),y_N^primeprime=y_N^(2),ldots, x_N^prime,x_N^primeprime,ldots, j_N^prime,j_N^primeprime,ldots] are obtained on replacing n_k in the above definitions by n_k^{prime},n_k^{primeprime},ldots; they can be entered as

[beginarraylll EY(N,d) & = & y_N^(d)
EX(N,d) & = & x_N^(d)
& vdots
EJ(N,d) & = & j_n^(d)

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

## 13.13 StarCyc

StarCyc( z )

If z is an irrational element of a quadratic number field (i.e. if z is a quadratic irrationality), StarCyc( z ) returns the unique Galois conjugate of z that is different from z; this is often called <z>ast (see DisplayCharTable). Otherwise false is returned.

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

If z is a cyclotomic integer that is contained in a quadratic number field over the rationals, it can be written as <z> = frac{ a + b sqrt{n} }{d} with integers a, b, n and d, where d is either 1 or 2. In this case Quadratic( z ) returns a record with fields a, b, root, d and ATLAS where the first four mean the integers mentioned above, and the last one is a string that is a (not necessarily shortest) representation of z by b_m, i_m or r_m for m = '|root|' (see ATLAS irrationalities).

If z is not a quadratic irrationality or not a cyclotomic integer, false is returned.

rec(
a := -1,
b := 1,
root := 5,
d := 2,
ATLAS := "b5" )
rec(
a := -1,
b := 3,
root := -3,
d := 2,
ATLAS := "1+3b3" )
rec(
a := 0,
b := 0,
root := 1,
d := 1,
ATLAS := "0" )
false

## 13.15 GaloisMat

GaloisMat( mat )

mat must be a matrix of cyclotomics (or possibly unknowns, see Unknown). The conjugate of a row in mat under a particular Galois automorphism is defined pointwise. If mat consists of full orbits under this action then the Galois group of its entries acts on mat as a permutation group, otherwise the orbits must be completed before.

GaloisMat( mat ) returns a record with fields mat, galoisfams and generators:

mat:

a list with initial segment mat (not a copy of mat); the list consists of full orbits under the action of the Galois group of the entries of mat defined above. The last entries are those rows which had to be added to complete the orbits; so if they were already complete, mat and mat have identical entries.

galoisfams:

a list that has the same length as mat; 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; in this case [ 'mat[i]' ] is regarded as an orbit, too, even if mat[i] contains irrational entries.
If 'galoisfams[i]' = [ l_1, l_2 ] is a list then mat[i] is the first element of its orbit in mat; l_1 is the list of positions of rows which form the orbit, and l_2 is the list of corresponding Galois automorphisms (as exponents, not as functions); so we have 'mat'[ l_1[j] ][k] = 'GaloisCyc'( 'mat'[i][k], l_2[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.

Note that mat should be a set, i.e. no two rows should be equal. Otherwise only the first row of some equal rows is considered for the permutations, and a warning is printed.

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> 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) ] )

## 13.16 RationalizedMat

RationalizedMat( mat )

returns the set of rationalized rows of mat, i.e. the set of sums over orbits under the action of the Galois group of the elements of mat (see GaloisMat).

This may be viewed as a kind of trace operation for the rows.

Note that mat should be a set, i.e. no two rows should be equal.

gap> mat:= CharTable( "A5" ).irreducibles;
[ [ 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 ] ]

GAP 3.4.4
April 1997