Singular in Sage. MCS 507 Lecture 40 Mathematical, Statistical and Scientific Software Jan Verschelde, 23 November 2011

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1 bases Singular in Sage 1 2 bases MCS 507 Lecture 40 Mathematical, Statistical Scientific Software Jan Verschelde, 23 November 2011

2 Singular in Sage bases 1 2 bases

3 bases Singular Singular is a computer algebra system for, with special emphasis on commutative non-commutative algebra, algebraic geometry, singularity theory, under the GNU General Public License. Singular s core algorithms hle factorization resultants characteristic sets numerical root finding, stard bases, free resolutions. Its development is directed by Wolfram Decker, Gert-Martin Greuel, Gerhard Pfister, Hans Schönemann, within the Dept. of Mathematics at the University of Kaiserslautern. page 3

4 bases Singular in Sage Advanced algorithms are contained in more than 90 libraries, written in a C-like programming language. For visualization of plane curves surfaces, the software surf surfex need to be installed. We run Singular explicitly in Sage via 1 the class Singular, do help(singular); or 2 opening a Terminal Session with Singular, type singular_console(); in a Sage terminal. page 4

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6 bases rings of s Before we enter a in Singular, we must first decide on 1 the type of the coefficients, 2 names order of the variables, 3 which order will be used for the monomials. In a Singular console, at the prompt > we type > ring r = 0, (x,y), Dp; We declared a ring r over the rational numbers (characteristic 0), for s in x y, in the degree lexicographic ordering (as in a dictionary). page 6

7 bases entering s > poly p = 4*y^ *x*y + x^2; > p; x2+4xy+4y2-4 We see p shown in the degree lexicographic order: first the highest degree monomials (x2 = x 2 ), with those with highest powers of x appearing first. Then we define a second q: > poly q = 4*x^2-4*x*y + y^2-4; The s p q define a system. page 7

8 bases > p; x2+4xy+4y2-4 > q; 4x2-4xy+y2-4 elimination with a resultant To eliminate x from the system p(x,y) = 0 = q(x,y): > poly rx = resultant(p,q,x); > rx; 625y4-1000y2+144 All values for y for which p(x,y) = 0 = q(x,y) also make resultant(p,q,x) zero. page 8

9 bases loading a library > LIB "solve.lib"; // ** loaded /usr/local/sage-4.7.1/local/share/\ singular/solve.lib (12231, ) // **../singular/triang.lib (12231, ) // **../singular/elim.lib (12231, ) // **../singular/ring.lib (12231, ) // **../singular/primdec.lib (12962, ) // **../singular/absfact.lib (12231, ) // **../singular/matrix.lib (12898, ) // **../singular/nctools.lib (12790, ) // **../singular/rom.lib (12827, ) // **../singular/poly.lib (12443, ) // **../singular/inout.lib (12541, ) // **../singular/general.lib (12904, ) page 9

10 bases We ask to compute all roots with 10 decimal places of precision: computing all roots > list L = laguerre_solve(rx,10); > L; [1]: -1.2 [2]: -0.4 [3]: 0.4 [4]: 1.2 page 10

11 bases Root finding is related to factorization: factorization > LIB "factor.lib"; // ** loaded /usr/local/sage-4.7.1/local/share/\ singular/factor.lib (12231, ) > factorize(rx); [1]: _[1]=1 _[2]=5y+2 _[3]=5y-2 _[4]=5y-6 _[5]=5y+6 [2]: 1,1,1,1,1 The factors give the exact roots: ± 2 5, ± 6 5. page 11

12 bases > solve([p,q]); [1]: [1]: 0.4 [2]: -1.2 [2]: [1]: -1.2 [2]: -0.4 [3]: [1]: 1.2 [2]: 0.4 [4]: [1]: -0.4 [2]: 1.2 solving a system The four solutions are (+0.4, 1.2), ( 1.2, 0.4), ( 0, 4, +1.2), (+1.2, +0.4). page 12

13 bases an explanation // solve created a ring, in which a list SOL // of numbers (the complex solutions) is stored. // To access the list of complex solutions, // type (if the name R was assigned // to the return value): setring R; SOL; // characteristic : 0 (complex:8 digits, // additional 8 digits) // 1 parameter : i // minpoly : (i^2+1) // number of vars : 2 // block 1 : ordering lp // : names x y // block 2 : ordering C page 13

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15 bases bases for ideals Given a set S of s, we call the set of all combinations of the s in S an ideal. For p q (as before), we define the ideal I as > ideal I = p,q; A basis solves the membership problem: Given any f, does f belong to I? > ideal g = groebner(i); > g; g[1]=20xy+15y2-12 g[2]=4x2-4xy+y2-4 g[3]=125y3+48x-164y page 15

16 bases changing rings A lexicographic monomial order eliminates variables the resulting basis has a triangular form. A lexicographic monomial order in Singular is lp: > ring s = 0, (x,y), lp; After this ring definition, the current ring becomes s the ideal I does not exist in this ring. To return to the original ring r, we type > setring r; page 16

17 bases reduced stard basis After returning to the ring r, we have > I; I[1]=x2+4xy+4y2-4 I[2]=4x2-4xy+y2-4 Before converting to a lexicographic order, we must force the computation of a reduced stard basis: > option(redsb); > I = std(i); > I; I[1]=20xy+15y2-12 I[2]=5x2+5y2-8 I[3]=125y3+48x-164y page 17

18 bases converting to lex order Instead of computing a basis in the ring s from scratch, we convert with the FGLM Algorithm: > setring s; > ideal J = fglm(r,i); > J; J[1]=625y4-1000y2+144 J[2]=48x+125y3-164y We see that after conversion we get a triangular form. page 18

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20 > setring r; > g; g[1]=20xy+15y2-12 g[2]=4x2-4xy+y2-4 g[3]=125y3+48x-164y normal forms bases With a basis g we can rewrite any modulo the ideal into a unique normal form: > reduce(x^2,g); -y2+8/5 > reduce(x*y,g); -3/4y2+3/5 > reduce(x*y^2,g); 36/125x-48/125y Monomials returned by reduce are 1, x, y, y 2. page 20

21 bases the library rootsmr.lib For a system with finitely many solutions, say N, the number of different monomials that can appear in the outcome of reduce equals N. > LIB "rootsmr.lib"; // ** loaded /usr/local/sage-4.7.1/local/share/\ singular/rootsmr.lib // **../singular/rootsur.lib // **../singular/linalg.lib (12258, ) With the rootsmr.lib written by Enrique A. Tobis, we can compute the generalization of a companion matrix for several variables. page 21

22 bases the quotient ring A basis for the quotient ring is computed as > ideal B = qbase(i); > B; B[1]=y2 B[2]=x B[3]=y B[4]=1 Because the system defined by p q has four solutions, the computation modulo p q defines a four dimensional quotient ring. page 22

23 bases > matrix m = matmult(x,b,i); > print(m); 0, -1, -3/4,0, 36/125, 0, 0, 1, -48/125,0, 0, 0, 0, 8/5,3/5, 0 a matrix The columns of the matrix m contain the normal forms of the multiples of the basis elements in B with x. Recall > reduce(x*y^2,g); 36/125x-48/125y We constructed the eigenvalue problem x B = m B. page 23

24 bases eigenvalues of m The eigenvalues of the matrix give the values for the x coordinate of the solutions. > eigenvals(m); [1]: _[1]=-6/5 _[2]=-2/5 _[3]=2/5 _[4]=6/5 [2]: 1,1,1,1 To avoid introducing multiplicities, one should take a rom linear form instead of multiplying with x. page 24

25 bases Summary + Exercises In addition to the online manual, there is A Singular Introduction to Commutative Algebra by Gert-Martin Greuel Gerhard Pfister, 2nd edition, Springer-Verlag, Compute a resultant from within a Sage session. 2 Install the software surf surfex explore the plotting of algebraic plane curves surfaces. Final Project due Friday 9 December at 10:30AM. page 25

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