Benchmark test e^x^3

[Albert Chan]

Using XCas 1.4.9-57 (win32), try to see the closed form integral

y := integrate(exp(x^3), x, 0, n)
==> No check were made for singular points of antiderivative ...
==> - igamma(1/3, -n^3) / 3

Ignoring the warning, and no clue what igamma does:

subst(y, n=6.0) => 5.96393809189e+91 // ok

subst(y, n=7.0) => 6.25946729139e+146 // close
integrate(exp(x^3), x, 0, 7.0) => 6.25946729142e+146

subst(y, n=8.0) => 7.91127730599e+154 // bad
integrate(exp(x^3), x, 0, 8.0) => 1.19135302958e+220

How to check if the result is right ? (last one were off by a lot)
What does igamma function do ?