Problem 11612
(American Mathematical Monthly, Vol.118, December 2011) Proposed by Paul Bracken (USA).
Evaluate in closed form
∞
Y
n=1
n + z + 1 n + z
n
e(2z−2n+1)/(2n).
Solution proposed by Roberto Tauraso, Dipartimento di Matematica, Universit`a di Roma “Tor Vergata”, via della Ricerca Scientifica, 00133 Roma, Italy.
Let z ∈ C \ {−1, −2, −3, . . . }. Since
Γ(z + 1) ∼ N ! exp(z log N ) QN
n=1(n + z) ∼
√2π exp(N log N + (z + 1/2) log N − N ) QN
n=1(n + z) ,
it follows that
N
Y
n=1
n + z + 1 n + z
n
=NN 1 +1+zN N
QN
n=1(n + z)
∼Γ(z + 1) exp(N log N + 1 + z − (N log N + (z + 1/2) log N − N ))
√ 2π
∼Γ(z + 1) exp(z + 1 − (z + 1/2) log N + N ))
√2π .
Hence
N
Y
n=1
n + z + 1 n + z
n
e(2z−2n+1)/(2n)∼Γ(z + 1) exp(z + 1 − (z + 1/2) log N + N + (z + 1/2)HN − N )
√2π
∼Γ(z + 1) exp(z + 1 − (z + 1/2) log N + N + (z + 1/2)(log N + γ) − N )
√2π
∼Γ(z + 1) exp(z + 1 + γ(z + 1/2))
√ 2π where we used the fact that HN =PN
n=11/n ∼ log N + γ.