14 Integers

These global variables represent the ring of integers and the semirings of positive and nonnegative integers, respectively.

gap> Size( Integers ); 2 in Integers;
infinity
true

are the defining categories for the domains Integers, PositiveIntegers, and NonnegativeIntegers.

gap> IsIntegers( Integers );  IsIntegers( Rationals );  IsIntegers( 7 );
true
false
false

Integers is a subset of Rationals, which is a subset of Cyclotomics. See Chapter Cyclotomic Numbers for arithmetic operations and comparison of integers.

14.1 Elementary Operations for Integers

Every rational integer lies in the category IsInt, which is a subcategory of IsRat (see Rational Numbers).

Every positive integer lies in the category IsPosInt.

Int returns an integer int whose meaning depends on the type of elm.

If elm is a rational number (see Rational Numbers) then int is the integer part of the quotient of numerator and denominator of elm (see QuoInt).

If elm is an element of a finite prime field (see Chapter Finite Fields) then int is the smallest nonnegative integer such that elm = int * One( elm ).

If elm is a string (see Chapter Strings and Characters) consisting of digits '0', '1', ¼, '9' and '-' (at the first position) then int is the integer described by this string. The operation String (see String) can be used to compute a string for rational integers, in fact for all cyclotomics.

gap> Int( 4/3 );  Int( -2/3 );
1
0
gap> int:= Int( Z(5) );  int * One( Z(5) );
2
Z(5)
gap> Int( "12345" );  Int( "-27" );  Int( "-27/3" );
12345
-27
fail

tests if the integer n is divisible by 2.

tests if the integer n is not divisible by 2.

AbsInt returns the absolute value of the integer n, i.e., n if n is positive, -n if n is negative and 0 if n is 0.

AbsInt is a special case of the general operation EuclideanDegree see EuclideanDegree).

See also AbsoluteValue.

gap> AbsInt( 33 );
33
gap> AbsInt( -214378 );
214378
gap> AbsInt( 0 );
0

SignInt returns the sign of the integer n, i.e., 1 if n is positive, -1 if n is negative and 0 if n is 0.

gap> SignInt( 33 );
1
gap> SignInt( -214378 );
-1
gap> SignInt( 0 );
0

LogInt returns the integer part of the logarithm of the positive integer n with respect to the positive integer base, i.e., the largest positive integer exp such that base^exp £ n. LogInt will signal an error if either n or base is not positive.

For base 2 this is very efficient because the internal binary representation of the integer is used.

gap> LogInt( 1030, 2 );
10        # 2^10 = 1024
gap> LogInt( 1, 10 );
0

RootInt returns the integer part of the kth root of the integer n. If the optional integer argument k is not given it defaults to 2, i.e., RootInt returns the integer part of the square root in this case.

If n is positive, RootInt returns the largest positive integer r such that r^k £ n. If n is negative and k is odd RootInt returns -RootInt( -n, k ). If n is negative and k is even RootInt will cause an error. RootInt will also cause an error if k is 0 or negative.

gap> RootInt( 361 );
19
gap> RootInt( 2 * 10^12 );
1414213
gap> RootInt( 17000, 5 );
7        # 7^5 = 16807

SmallestRootInt returns the smallest root of the integer n.

The smallest root of an integer n is the integer r of smallest absolute value for which a positive integer k exists such that n = r^k.

gap> SmallestRootInt( 2^30 );
2
gap> SmallestRootInt( -(2^30) );
-4        # note that $(-2)^{30} = +(2^{30})$
gap> SmallestRootInt( 279936 );
6
gap> LogInt( 279936, 6 );
7
gap> SmallestRootInt( 1001 );
1001

Random for integers returns pseudo random integers between -10 and 10 distributed according to a binomial distribution. To generate uniformly distributed integers from a range, use the construct 'Random( [ low .. high ] )'. (Also see Random.)

14.2 Quotients and Remainders

QuoInt returns the integer part of the quotient of its integer operands.

If n and m are positive QuoInt( n, m ) is the largest positive integer q such that q * m £ n . If n or m or both are negative the absolute value of the integer part of the quotient is the quotient of the absolute values of n and m, and the sign of it is the product of the signs of n and m.

QuoInt is used in a method for the general operation EuclideanQuotient (see EuclideanQuotient).

gap> QuoInt(5,3);  QuoInt(-5,3);  QuoInt(5,-3);  QuoInt(-5,-3);
1
-1
-1
1

BestQuoInt returns the best quotient q of the integers n and m. This is the quotient such that n-q*m has minimal absolute value. If there are two quotients whose remainders have the same absolute value, then the quotient with the smaller absolute value is chosen.

gap> BestQuoInt( 5, 3 );  BestQuoInt( -5, 3 );
2
-2

RemInt returns the remainder of its two integer operands.

If m is not equal to zero RemInt( n, m ) = n - m * QuoInt( n, m ). Note that the rules given for QuoInt imply that RemInt( n, m ) has the same sign as n and its absolute value is strictly less than the absolute value of m. Note also that RemInt( n, m ) = n mod m when both n and m are nonnegative. Dividing by 0 signals an error.

RemInt is used in a method for the general operation EuclideanRemainder (see EuclideanRemainder).

gap> RemInt(5,3);  RemInt(-5,3);  RemInt(5,-3);  RemInt(-5,-3);
2
-2
2
-2

GcdInt returns the greatest common divisor of its two integer operands m and n, i.e., the greatest integer that divides both m and n. The greatest common divisor is never negative, even if the arguments are. We define GcdInt( m, 0 ) = GcdInt( 0, m ) = AbsInt( m ) and GcdInt( 0, 0 ) = 0.

GcdInt is a method used by the general function Gcd (see Gcd).

gap> GcdInt( 123, 66 );
3

returns a record g describing the extended greatest common divisor of m and n. The component gcd is this gcd, the components coeff1 and coeff2 are integer cofactors such that g.gcd = g.coeff1 * m + g.coeff2 * n, and the components coeff3 and coeff4 are integer cofactors such that 0 = g.coeff3 * m + g.coeff4 * n.

If m and n both are nonzero, AbsInt( g.coeff1 ) is less than or equal to AbsInt(n) / (2 * g.gcd) and AbsInt( g.coeff2 ) is less than or equal to AbsInt(m) / (2 * g.gcd).

If m or n or both are zero coeff3 is -n / g.gcd and coeff4 is m / g.gcd.

The coefficients always form a unimodular matrix, i.e., the determinant g.coeff1 * g.coeff4 - g.coeff3 * g.coeff2 is 1 or -1.

gap> Gcdex( 123, 66 );
rec( gcd := 3, coeff1 := 7, coeff2 := -13, coeff3 := -22, coeff4 := 41 )
  # 3 = 7*123 - 13*66, 0 = -22*123 + 41*66
gap> Gcdex( 0, -3 );
rec( gcd := 3, coeff1 := 0, coeff2 := -1, coeff3 := 1, coeff4 := 0 )
gap> Gcdex( 0, 0 );
rec( gcd := 0, coeff1 := 1, coeff2 := 0, coeff3 := 0, coeff4 := 1 )

returns the least common multiple of the integers m and n.

LcmInt is a method used by the general function Lcm.

gap> LcmInt( 123, 66 );
2706

returns the q-adic representation of the integer i as a list l of coefficients where i = å_j=0 q^j ·l[j+1].

returns the multiadic expansion of the integer int modulo the integers given in ints (in ascending order). It returns a list of coefficients in the reverse order to that in ints.

ChineseRem returns the combination of the residues modulo the moduli, i.e., the unique integer c from [0..Lcm(moduli)-1] such that c = residues[i] modulo moduli[i] for all i, if it exists. If no such combination exists ChineseRem signals an error.

Such a combination does exist if and only if residues[i]=residues[k] mod Gcd(moduli[i],moduli[k]) for every pair i, k. Note that this implies that such a combination exists if the moduli are pairwise relatively prime. This is called the Chinese remainder theorem.

gap> ChineseRem( [ 2, 3, 5, 7 ], [ 1, 2, 3, 4 ] );
53
gap> ChineseRem( [ 6, 10, 14 ], [ 1, 3, 5 ] );
103

gap> ChineseRem( [ 6, 10, 14 ], [ 1, 2, 3 ] );
Error, the residues must be equal modulo 2 called from
... lines omitted here ...

returns r^e mod m for integers r,e and m (e ³ 0). Note that using r ^ e mod m will generally be slower, because it can not reduce intermediate results the way PowerModInt does but would compute r^e first and then reduce the result afterwards.

PowerModInt is a method for the general operation PowerMod.

14.3 Prime Integers and Factorization

Primes is a strictly sorted list of the 168 primes less than 1000.

This is used in IsPrimeInt and FactorsInt to cast out small primes quickly.

gap> Primes[1];
2
gap> Primes[100];
541

IsPrimeInt returns false if it can prove that n is composite and true otherwise. By convention IsPrimeInt(0) = IsPrimeInt(1) = false and we define IsPrimeInt( -n ) = IsPrimeInt( n ).

IsPrimeInt will return true for every prime n. IsPrimeInt will return false for all composite n < 10¹³ and for all composite n that have a factor p < 1000. So for integers n < 10¹³, IsPrimeInt is a proper primality test. It is conceivable that IsPrimeInt may return true for some composite n > 10¹³, but no such n is currently known. So for integers n > 10¹³, IsPrimeInt is a probable-primality test. Therefore IsPrimeInt will issue a warning when called with an argument > 10¹³. (The function IsProbablyPrimeInt will do the same calculations but not issue a warning.)

If composites that fool IsPrimeInt do exist, they would be extremely rare, and finding one by pure chance might be less likely than finding a bug in GAP. We would appreciate being informed about any example of a composite number n for which IsPrimeInt returns true.

IsPrimeInt is a deterministic algorithm, i.e., the computations involve no random numbers, and repeated calls will always return the same result. IsPrimeInt first does trial divisions by the primes less than 1000. Then it tests that n is a strong pseudoprime w.r.t. the base 2. Finally it tests whether n is a Lucas pseudoprime w.r.t. the smallest quadratic nonresidue of n. A better description can be found in the comment in the library file integer.gi.

The time taken by IsPrimeInt is approximately proportional to the third power of the number of digits of n. Testing numbers with several hundreds digits is quite feasible.

IsPrimeInt is a method for the general operation IsPrime.

gap> IsPrimeInt( 2^31 - 1 );
true
gap> IsPrimeInt( 10^42 + 1 );
false

IsPrimePowerInt returns true if the integer n is a prime power and false otherwise.

n is a prime power if there exists a prime p and a positive integer i such that pⁱ = n. If n is negative the condition is that there must exist a negative prime p and an odd positive integer i such that pⁱ = n. 1 and -1 are not prime powers.

Note that IsPrimePowerInt uses SmallestRootInt (see SmallestRootInt) and a probable-primality test (see IsPrimeInt).

gap> IsPrimePowerInt( 31^5 );
true
gap> IsPrimePowerInt( 2^31-1 );
true        # $2^{31}-1$ is actually a prime
gap> IsPrimePowerInt( 2^63-1 );
false
gap> Filtered( [-10..10], IsPrimePowerInt );
[ -8, -7, -5, -3, -2, 2, 3, 4, 5, 7, 8, 9 ]

NextPrimeInt returns the smallest prime which is strictly larger than the integer n.

Note that NextPrimeInt uses a probable-primality test (see IsPrimeInt).

gap> NextPrimeInt( 541 ); NextPrimeInt( -1 );
547
2

PrevPrimeInt returns the largest prime which is strictly smaller than the integer n.

Note that PrevPrimeInt uses a probable-primality test (see IsPrimeInt).

gap> PrevPrimeInt( 541 ); PrevPrimeInt( 1 );
523
-2

FactorsInt returns a list of prime factors of the integer n.

If the ith power of a prime divides n this prime appears i times. The list is sorted, that is the smallest prime factors come first. The first element has the same sign as n, the others are positive. For any integer n it holds that Product( FactorsInt( n ) ) = n.

Note that FactorsInt uses a probable-primality test (see IsPrimeInt). Thus FactorsInt might return a list which contains composite integers.

The time taken by FactorsInt is approximately proportional to the square root of the second largest prime factor of n, which is the last one that FactorsInt has to find, since the largest factor is simply what remains when all others have been removed. Thus the time is roughly bounded by the fourth root of n. FactorsInt is guaranteed to find all factors less than 10⁶ and will find most factors less than 10¹⁰. If n contains multiple factors larger than that FactorsInt may not be able to factor n and will then signal an error.

FactorsInt is used in a method for the general operation Factors.

In the second form, FactorsInt calls FactorsRho with a limit of trials on the number of trials is performs. The default is 8192.

gap> FactorsInt( -Factorial(6) );
[ -2, 2, 2, 2, 3, 3, 5 ]
gap> Set( FactorsInt( Factorial(13)/11 ) );
[ 2, 3, 5, 7, 13 ]
gap> FactorsInt( 2^63 - 1 );
[ 7, 7, 73, 127, 337, 92737, 649657 ]
gap> FactorsInt( 10^42 + 1 );
#I  beyond the guaranteed bound of the probabilistic primality test
[ 29, 101, 281, 9901, 226549, 121499449, 4458192223320340849 ]

prints the prime factorization of the integer n in human-readable form.

gap> PrintFactorsInt( Factorial( 7 ) ); Print( "\n" );
2^4*3^2*5*7

returns the prime factorization of the integer n as a list [ p₁, e₁, ¼, p_n, e_n ] with n = Õ_i=1ⁿ p_i^e_i.

gap> PrimePowersInt( Factorial( 7 ) );
[ 2, 4, 3, 2, 5, 1, 7, 1 ]

DivisorsInt returns a list of all divisors of the integer n. The list is sorted, so that it starts with 1 and ends with n. We define that Divisors( -n ) = Divisors( n ).

Since the set of divisors of 0 is infinite calling DivisorsInt( 0 ) causes an error.

DivisorsInt may call FactorsInt to obtain the prime factors. Sigma and Tau (see Sigma and Tau) compute the sum and the number of positive divisors, respectively.

gap> DivisorsInt( 1 ); DivisorsInt( 20 ); DivisorsInt( 541 );
[ 1 ]
[ 1, 2, 4, 5, 10, 20 ]
[ 1, 541 ]

14.4 Residue Class Rings

If r, s and n are integers, r / s as a reduced fraction is p / q, and q and n are coprime, then r / s mod n is defined to be the product of p and the inverse of q modulo n. See Section Arithmetic Operators for more details and definitions.

With the above definition, 4 / 6 mod 32 equals 2 / 3 mod 32 and hence exists (and is equal to 22), despite the fact that 6 has no inverse modulo 32.

ZmodnZ returns a ring R isomorphic to the residue class ring of the integers modulo the positive integer n. The element corresponding to the residue class of the integer i in this ring can be obtained by i * One( R ), and a representative of the residue class corresponding to the element x Î R can be computed by Int( x ).

ZmodnZ( n ) is equivalent to Integers mod n.

ZmodpZ does the same if the argument p is a prime integer, additionally the result is a field. ZmodpZNC omits the check whether p is a prime.

Each ring returned by these functions contains the whole family of its elements if n is not a prime, and is embedded into the family of finite field elements of characteristic n if n is a prime.

If the first argument is a residue class family Fam then ZmodnZObj returns the element in Fam whose coset is represented by the integer r. If the two arguments are an integer r and a positive integer n then ZmodnZObj returns the element in ZmodnZ( n ) (see ZmodnZ) whose coset is represented by the integer r.

gap> r:= ZmodnZ(15);
(Integers mod 15)
gap> fam:=ElementsFamily(FamilyObj(r));;
gap> a:= ZmodnZObj(fam,9);
ZmodnZObj( 9, 15 )
gap> a+a;
ZmodnZObj( 3, 15 )
gap> Int(a+a);
3

The elements in the rings Z / n Z are in the category IsZmodnZObj. If n is a prime then the elements are of course also in the category IsFFE (see IsFFE), otherwise they are in IsZmodnZObjNonprime. IsZmodpZObj is an abbreviation of IsZmodnZObj and IsFFE. This category is the disjoint union of IsZmodpZObjSmall and IsZmodpZObjLarge, the former containing all elements with n at most MAXSIZE_GF_INTERNAL.

The reasons to distinguish the prime case from the nonprime case are

• that objects in IsZmodnZObjNonprime have an external representation (namely the residue in the range [ 0, 1, ¼, n-1 ]),
• that the comparison of elements can be defined as comparison of the residues, and
• that the elements lie in a family of type IsZmodnZObjNonprimeFamily (note that for prime n, the family must be an IsFFEFamily).

The reasons to distinguish the small and the large case are that for small n the elements must be compatible with the internal representation of finite field elements, whereas we are free to define comparison as comparison of residues for large n.

Note that we cannot claim that every finite field element of degree 1 is in IsZmodnZObj, since finite field elements in internal representation may not know that they lie in the prime field.

The residue class rings are rings, thus all operations for rings (see Chapter Rings) apply. See also Chapters Finite fields and Number theory.

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GAP 4 manual
May 2002