OFFSET
1,2
COMMENTS
Denominators of the Eulerian numbers T(-2,k) for k = 0,1..., if T(n,k) is extended to negative n by the recurrence T(n,k) = (k+1)*T(n-1,k) + (n-k)*T(n-1,k-1) (indexed as in A173018). - Michael J. Collins, Oct 10 2024
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..1152 (terms 1..200 from T. D. Noe)
D. Y. Savio, E. A. Lamagna and S.-M. Liu, Summation of harmonic numbers, pp. 12-20 of E. Kaltofen and S. M. Watt, editors, Computers and Mathematics, Springer-Verlag, NY, 1989.
FORMULA
a(n) = denominator of (Pi^2)/6 - zeta(2, x). - Artur Jasinski, Mar 03 2010
EXAMPLE
1/1^2 + 1/2^2 + 1/3^2 = 1/1 + 1/4 + 1/9 = 49/36, so a(3) = 36. - Jon E. Schoenfield, Dec 26 2014
MAPLE
ZL:=n->sum(1/i^2, i=2..n): a:=n->floor(denom(ZL(n))): seq(a(n), n=1..21); # Zerinvary Lajos, Mar 28 2007
MATHEMATICA
s=0; lst={}; Do[s+=n^2/n^4; AppendTo[lst, Denominator[s]], {n, 3*4!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jan 24 2009 *)
Table[Denominator[Pi^2/6 - Zeta[2, x]], {x, 1, 22}] (* Artur Jasinski, Mar 03 2010 *)
Denominator[Accumulate[1/Range[30]^2]] (* Harvey P. Dale, Nov 08 2012 *)
PROG
(Haskell)
import Data.Ratio ((%), denominator)
a007407 n = a007407_list !! (n-1)
a007407_list = map denominator $
scanl1 (+) $ map (1 %) $ tail a000290_list
-- Reinhard Zumkeller, Jul 06 2012
(PARI) a(n)=denominator(sum(k=1, n, 1/k^2)) \\ Charles R Greathouse IV, Nov 20 2012
(Python)
from fractions import Fraction
def A007407(n): return sum(Fraction(1, k**2) for k in range(1, n+1)).denominator # Chai Wah Wu, Apr 03 2021
CROSSREFS
KEYWORD
nonn,easy,frac,nice
AUTHOR
STATUS
approved