This method computes a regular sequence of length n contained in a given ideal I. It attempts to do so by first trying "sparse" combinations of the generators, i.e. elements which are either generators or sums of two generators. If a sparse regular sequence is not found, then dense combinations of generators will be tried.
If the length n is either unspecified or greater than the codimension of I then it is silently replaced with the codimension of I. The ideal I should be in a polynomial (or at least Cohen-Macaulay) ring, so that codim I = grade I.
i1 : R = QQ[x_0..x_7] o1 = R o1 : PolynomialRing |
i2 : I = intersect(ideal(x_0,x_1,x_2,x_3), ideal(x_4,x_5,x_6,x_7), ideal(x_0,x_2,x_4,x_6), ideal(x_1,x_3,x_5,7))
o2 = ideal (x x , x x , x x , x x , x x , x x , x x , x x , x x , x x , x x ,
2 7 0 7 3 6 2 6 1 6 0 6 2 5 0 5 3 4 2 4 1 4
------------------------------------------------------------------------
x x )
0 4
o2 : Ideal of R
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i3 : elapsedTime regSeqInIdeal I
-- 0.0545965 seconds elapsed
o3 = ideal (x x , x x + x x , x x + x x , x x + x x )
2 7 3 6 0 7 2 5 0 7 1 4 0 7
o3 : Ideal of R
|
If I is the unit ideal, then an ideal of variables of the ring is returned.
If the codimension of I is already known, then one can specify this, along with a time limit for each trial (normally this is taken from the length of time for computing codim I). This can result in a significant speedup: in the following example, codim I takes more than a minute to complete.
i4 : R = QQ[h,l,s,x,y,z] o4 = R o4 : PolynomialRing |
i5 : I = ideal(h*l-l^2-4*l*s+h*y,h^2*s-6*l*s^3+h^2*z,x*h^2-l^2*s-h^3,h^8,l^8,s^8)
2 3 2 2 3 2 2 8
o5 = ideal (h*l - l - 4l*s + h*y, - 6l*s + h s + h z, - h - l s + h x, h ,
------------------------------------------------------------------------
8 8
l , s )
o5 : Ideal of R
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i6 : isSubset(I, ideal(s,l,h)) -- implies codim I == 3 o6 = true |
i7 : elapsedTime regSeqInIdeal(I, 3, 3, 1)
-- 0.0133678 seconds elapsed
2 3 2 2 8 3 2 2
o7 = ideal (h*l - l - 4l*s + h*y, h + l s - h x, s + h + l s - h x)
o7 : Ideal of R
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The object regSeqInIdeal is a method function with options.