2
$\begingroup$

Let $$y=\sum_{n=1}^{x}n!$$ be the sum of consecutive factorials.

What is closed form for $y$ in terms of $x$? Wolfram Alpha says that $$y=-(-1)^x\Gamma(x+2)(!(-x-2))-!(-1)-1$$ where $!x$ is subfactorial function of $x$, but there isn't any step-by-step method available. Can you explain how solve this?

$\endgroup$
  • 1
    $\begingroup$ Have you tried induction on $x$? $\endgroup$ – Frunobulax Aug 13 '14 at 14:39
  • 1
    $\begingroup$ For others reading this, note that $!x$ is not a typo but rather the so-called subfactorial. (It would be helpful to put that into your question.) $\endgroup$ – Semiclassical Aug 13 '14 at 14:59
  • $\begingroup$ see this and this. $\endgroup$ – aziiri Aug 13 '14 at 15:12
  • $\begingroup$ Given @aziiri's first link, this is essentially a duplicate question. But I don't find myself terribly satisfied by the answers in those questions. $\endgroup$ – Semiclassical Aug 13 '14 at 15:15
  • $\begingroup$ Me too, the result is quite ugly. But this the case of many finite sums, for example the harmonic sum also have no beautiful closed form (some will consider $H_n$ a closed form). $\endgroup$ – aziiri Aug 13 '14 at 15:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy