Problem 12220
(American Mathematical Monthly, Vol.127, December 2020) Proposed by D. M. Batinetu-Giurgiu and N. Stanciu (Romania).
Letan =Pn
k=11/k2andbn=Pn
k=11/(2k − 1)2. Prove
n→∞lim n bn
an −3 4
= 3 π2.
Solution proposed by Roberto Tauraso, Dipartimento di Matematica, Universit`a di Roma “Tor Vergata”, via della Ricerca Scientifica, 00133 Roma, Italy.
Solution. Since bn= a2n−an
4 , as n → ∞, n bn
an −3 4
= n a2n an −1
= n an
n
X
k=1
1
(n + k)2 = 1 an · 1
n
n
X
k=1
1
(1 + k/n)2 → 3 π2 because
an →π2
6 and 1 n
n
X
k=1
1
(1 + k/n)2 → Z 1
0
dx (1 + x)2 = 1
2.