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Macaulay2Doc :: methods for normal forms and remainder

methods for normal forms and remainder -- normal form of ring elements and matrices

Synopsis

To reduce f with respect to I, a (partial) Gröbner basis of I is computed, unless it has already been done, or unless I is a monomial ideal.

i1 : R = ZZ/1277[x,y];
i2 : I = ideal(x^3 - 2*x*y, x^2*y - 2*y^2 + x);

o2 : Ideal of R
i3 : (x^3 - 2*x) % I

o3 = -2x

o3 : R
i4 : (x^3) % I

o4 = 0

o4 : R
i5 : S = ZZ[x,y];
i6 : 144*x^2*y^2 % (7*x*y-2)

         2 2
o6 = - 3x y  + 12

o6 : S

Synopsis

  • Usage:
    f % M
  • Inputs:
  • Outputs:
    • a matrix, the normal form of the matrix f modulo the image of M, if M is a matrix with the same target as f, or modulo M if M is a submodule of the target of f. The result is obtained by reducing each column of f modulo M.
i7 : S = QQ[a..f]

o7 = S

o7 : PolynomialRing
i8 : J = ideal(a*b*c-d*e*f,a*b*d-c*e*f, a*c*e-b*d*f)

o8 = ideal (a*b*c - d*e*f, a*b*d - c*e*f, a*c*e - b*d*f)

o8 : Ideal of S
i9 : C = res J

      1      3      6      6      2
o9 = S  <-- S  <-- S  <-- S  <-- S  <-- 0
                                         
     0      1      2      3      4      5

o9 : ChainComplex
i10 : F = syz transpose C.dd_4

o10 = {-8} | 0 0 a de  ce bd  bc  0   0   0  0   |
      {-8} | 0 b 0 df  cf 0   0   ad  ac  0  0   |
      {-8} | c 0 0 -ef 0  -bf 0   -ae 0   ab 0   |
      {-8} | d 0 0 0   ef 0   bf  0   ae  0  ab  |
      {-8} | 0 e 0 0   0  -df -cf 0   0   ad -ac |
      {-8} | 0 0 f 0   0  0   0   -de -ce bd -bc |

              6       11
o10 : Matrix S  <--- S
i11 : G = transpose C.dd_3

o11 = {-8} | bd  -ce de  a 0  0  |
      {-8} | -ac -cf df  0 -b 0  |
      {-8} | -bf -ab -ef 0 0  -c |
      {-8} | -ae -ef -ab 0 0  -d |
      {-8} | -df -ad ac  0 -e 0  |
      {-8} | ce  -bd bc  f 0  0  |

              6       6
o11 : Matrix S  <--- S

Since C is a complex, we know that the image of G is contained in the image of F.

i12 : G % F

o12 = 0

              6       6
o12 : Matrix S  <--- S
i13 : F % G

o13 = {-8} | 0 0 0 de  ce bd  bc  0   bd  -ce de  |
      {-8} | 0 0 0 df  cf 0   0   ad  0   -cf df  |
      {-8} | 0 0 0 -ef 0  -bf 0   -ae -bf 0   -ef |
      {-8} | 0 0 0 0   ef 0   bf  0   0   -ef 0   |
      {-8} | 0 0 0 0   0  -df -cf 0   -df 0   0   |
      {-8} | 0 0 0 0   0  0   0   -de 0   0   0   |

              6       11
o13 : Matrix S  <--- S

The inclusion is strict, since F % G != 0 implies that the image of F is not contained in the image of G.

Normal forms work over quotient rings too.

i14 : A = QQ[x,y,z]/(x^3-y^2-3)

o14 = A

o14 : QuotientRing
i15 : I = ideal(x^4, y^4)

                2        4
o15 = ideal (x*y  + 3x, y )

o15 : Ideal of A
i16 : J = ideal(x^3*y^2, x^2*y^3)

              4     2   2 3
o16 = ideal (y  + 3y , x y )

o16 : Ideal of A
i17 : (gens J) % I

o17 = 0

              1       2
o17 : Matrix A  <--- A

Here is an example involving rational functions.

i18 : kk = frac(ZZ[a,b])

o18 = kk

o18 : FractionField
i19 : B = kk[x,y,z]

o19 = B

o19 : PolynomialRing
i20 : I = ideal(a*x^2-b*x-y-1, 1/b*y^2-z-1)

                2                1 2
o20 = ideal (a*x  - b*x - y - 1, -y  - z - 1)
                                 b

o20 : Ideal of B
i21 : gens gb I

o21 = | y2-bz-b x2+(-b)/ax+(-1)/ay+(-1)/a |

              1       2
o21 : Matrix B  <--- B
i22 : x^2*y^2 % I

       2              2
      b       b      b     b    b    b
o22 = --x*z + -y*z + --x + -y + -z + -
       a      a       a    a    a    a

o22 : B

See also