Bill Allombert on Mon, 27 Feb 2006 19:29:56 +0100

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Re: qflllgram

On Mon, Feb 27, 2006 at 12:58:22PM -0500, Igor Schein wrote:
> I also have a question stemming from this behavior.  Is the first
> argument a valid input?  It's definitely symmetric, but does it

[1,0;0,-1] is certainly not definite: it is the form X^2-Y^2.
so this is not a valid input.

> correspond to a positive definite quadratic form, like the manual
> requires?  In general, what's the command to convert t_QFI to t_MAT
> and vice versa?  If it's answered somewhere in the manual, then I am
> sorry for not finding it myself; if it isn't, it might be FAQ-worthy.

The Gram matrix [a,b;b,c] correspond to the form Qfb(a,2*b,c).

You can use qfsign(M)[1]==#M to check whether a Gram matrix correspond to 
a definite positive form.