Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".
Zeta(2)=A013661 is 1 plus a sum over inverse squares of k-almost primes, k=1 to infinity, where A085548 represents k=1, A117543 represents k=2 and this constant here represents k=3.
Table of n, a(n) for n=0..104.
R. J. Mathar, Series of reciprocal powers of k-almost primes, arXiv:0803.0900 [math.NT].
Equals A085548^3 / 6 + A085966 / 3 + A085548 * A085964 / 2. Equals sum_(i=1,2,...infinity) 1/A000290[A014612(i)].
Cf. A117543, A085548.
Sequence in context: A016670 A021726 A221315 * A011239 A261644 A200699
Adjacent sequences: A131650 A131651 A131652 * A131654 A131655 A131656
R. J. Mathar, Sep 10 2007, Mar 07 2008