
D.14.1.2 signatureLqf
Procedure from library phindex.lib (see phindex_lib).
 Usage:
 signatureLqf(h); h quadratic form (poly type).
 Return:
 the signature of h of type int or if r is given and !=0 then
intvec with (signature, nr. of +, nr. of ) is returned.
 Theory:
 To compute the signature we use the method of Lagrange. The law of
inertia for a real quadratic form h(x,x) says that in a
representation of h(x,x) as a sum of independent squares
h(x,x)=sum_{i=1}^r a_i*X_i^2 the number of positive and the number of negative squares are
independent of the choice of representation. The signature s of
h(x,x) is the difference between the number pi of positive squares
and the number nu of negative squares in the representation of
h(x,x). The rank r of h(x,x) and the signature s determine the
numbers pi and nu uniquely, since
r=pi+nu, s=pinu.
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(1..4)),ds;
poly Ax=4*x(1)^2+x(2)^2+x(3)^2+x(4)^24*x(1)*x(2)4*x(1)*x(3)+4*x(1)*x(4)+4*x(2)*x(3)4*x(2)*x(4);
signatureLqf(Ax,1); //The rank of Ax is 3+1=4
==> 2,3,1
poly Bx=2*x(1)*x(4)+x(2)^2+x(3)^2;
signatureLqf(Bx);
==> 2

