"ati " <nati02@tonline.hu> wrote in message <holgpt$3fk$1@fred.mathworks.com>...
> Hi,
> I'd like to evaluate double improper intagral, like:
> >>F=@(t,x)exp(x.*t)./t.^3;
>
> >>dblquad(F,1,inf,0,inf));
> results: Warning: Infinite or NotaNumber function value encountered. ans= NaN
>
> and
> >> int(int(F,t,1,inf),x,0,inf);
> results: ??? Undefined function or variable 't'.
>
> Help is much appreciated.
You can try a change of variables that makes both limits finite to see if that helps. For example, try u = 1/t and y = 1/(x+1). That brings both u and y into the finite range from 0 to 1, which would avoid that first warning you received.
Unfortunately the transformed integrand becomes
u/y^2*exp((y1)/(u*y))
and you might receive a different warning if 'dblquad' evaluates this for values of u and y that are too close to zero. If that happens, you could fudge the two limits up just a very small amount away from zero to avoid such computational difficulties. The integrand values would actually be exceedingly small for such values of u and y in spite of the division by y^2.
As for attempting to find a symbolic integral with 'int', my (admittedly rather ancient) integral table lists only a power series for that particular integral, so I suspect that you would receive a matlab message to the effect that there was "no explicit solution found".
Roger Stafford
