im havin trouble evaluating integral [tex]\int_0^\infty\frac{x^{3}}{e^x-1}dx[/tex]

i tried integrating by substitution but got nowhere. if there wasnt factor -1 it would equal G(4) which is 3! but first integral equals pii^4/15 which is 6,49....

But any hint how to do this integral?

i tried integrating by substitution but got nowhere. if there wasnt factor -1 it would equal G(4) which is 3! but first integral equals pii^4/15 which is 6,49....

But any hint how to do this integral?

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