distribucions de probabilitat en series

f(k) = (x^{k}/k!)·e^{-x}
f(k) = k·(x^{k}/k!)·(1/x)·e^{-x}
f(k) = k(k+(-1))·(x^{k}/k!)·(1/x^{2})·e^{-x}
f(k) = k(k+(-1))·...(m)...·(k+(-m))·(x^{k}/k!)·(1/x^{m+1})·e^{-x}


f(k) = (x^{2k}/(2k)!)·(1/cosh(x))
f(k) = (x^{2k+1}/(2k+1)!)·(1/sinh(x))


f(k) = (2k)(x^{2k}/(2k)!)·(1/x)·(1/sinh(x))
f(k) = (2k+1)(x^{2k+1}/(2k+1)!)·(1/x)·(1/cosh(x))