| #### D.14.6.1 signatureL Procedure from library `phindex.lib` (see phindex_lib). Usage: signatureL(M[,r]); M symmetric matrix, r int (optional). Return: the signature of M of type int or if r is given and !=0 then intvec with (signature, nr. of +, nr. of -) is returned. Theory: Given the matrix M, we construct the quadratic form associated. Afterwards we use the method of Lagrange to compute the signature. The law of inertia for a real quadratic form A(x,x) says that in a representation of A(x,x) as a sum of independent squares A(x,x)=sum_{i=1}^r a_iX_i^2. The number of positive and the number of negative squares are independent of the choice of representation. The signature -s- of A(x,x) is the difference between the number -pi- of positive squares and the number -nu- of negative squares in the representation of A(x,x). The rank -r- of M (or A(x,x)) and the signature -s- determine the numbers -pi- and -nu- uniquely, since r=pi+nu, s=pi-nu. The method of Lagrange is a procedure to reduce any real quadratic form to a sum of squares. Ref. Gantmacher, The theory of matrices, Vol. I, Chelsea Publishing Company, NY 1960, page 299. Example:
    LIB "phindex.lib";
    ring r=0,(x),ds;
    matrix M[5][5]=0,0,0,1,0,0,1,0,0,-1,0,0,1,0,0,1,0,0,3,0,0,-1,0,0,1;
    signatureL(M,1); //The rank of M is 3+1=4
    ==> 2,3,1
    matrix H[5][5]=0,-7,0,1,0,-7,1,0,0,-1,0,0,1,0,0,1,0,0,-3,5,0,-1,0,5,1;
    signatureL(H);
    ==> 1

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