1,1

a(n) = 2 for n = {2,3,5,7,13,17,19,31,61,89,107,127,...} = A000043 Primes p such that 2^p - 1 is prime. 2^p - 1 is then called a Mersenne prime. a(n) = 3 for n = {1,4,10,12,24,27,39,...}. a(n) = 5 for n = {26,76,132,205,238,...}. a(n) = 9 for n = {100,200,...}. a(n) = 15 for n = {15,33,65,...}. a(21) = 18. a(22) = 13. a(41) = 6. a(72) = 11. a(173) = 8.

Eric Weisstein, Link to a section of The World of Mathematics. Harmonic Number.

Cf. A058313, A119682, A120296, A001008.

Cf. A000043.

Sequence in context: A127807 A122028 A049234 this_sequence A069901 A075392 A115039

Adjacent sequences: A125501 A125502 A125503 this_sequence A125505 A125506 A125507

hard,more,nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Dec 28 2006, Jan 31 2007