1,2

Denominators are A118392. Fractions are: 1/1, 5/4, 27/20, 7/5, 10/7, 81/56, 35/24, 22/15, 81/55, 65/44, 77/52, 135/91, 52/35, 119/80, 405/272, 76/51, 85/57, 567/380, 209/140, 115/77, 378/253, 275/184, 299/200, 486/325, 175/117, 377/252, 1215/812, 217/145, 232/155, 1485/992. The numerator of sum of reciprocals of first n triangular numbers is A022998.

2n+3 divides a(2n). 2n-1 divides a(2n-1). p divides a(p) for prime p>2. The only primes in a(n) are a(2) = 5 and a(4) = 7. - Alexander Adamchuk (alex(AT)kolmogorov.com), May 08 2007

A118391(n)/A118392(n) = SUM[i=1..n] (1/A000292(n)). A118391(n)/A118392(n) = SUM[i=1..n] (1/C(n+2,3)). A118391(n)/A118392(n) = SUM[i=1..n] (1/(n*(n+1)*(n+2)/6)).

a(n) = Numerator[ 3n(n+3) / (2(n+1)(n+2)) ]. - Alexander Adamchuk (alex(AT)kolmogorov.com), May 08 2007

a(1) = 1 = numerator of 1/1.

a(2) = 5 = numerator of 5/4 = 1/1 + 1/4.

a(3) = 27 = numerator of 27/20 = 1/1 + 1/4 + 1/10.

a(4) = 7 = numerator of 7/5 = 1/1 + 1/4 + 1/10 + 1/20.

a(5) = 10 = numerator of 10/7 = 1/1 + 1/4 + 1/10 + 1/20 + 1/35.

a(20) = 115 = numerator of 115/77 = 1/1 + 1/4 + 1/10 + 1/20 + 1/35 + 1/56 + 1/84 + 1/120 + 1/165 + 1/220 + 1/286 + 1/364 + 1/455 + 1/560 + 1/680 + 1/816 + 1/969 + 1/1140 + 1/1330 + 1/1540.

Table[ Numerator[ 3n(n+3) / (2(n+1)(n+2)) ], {n, 1, 100} ] - Alexander Adamchuk (alex(AT)kolmogorov.com), May 08 2007

Cf. A000292, A022998, A118392.

Sequence in context: A132509 A064489 A081089 this_sequence A097088 A091721 A039283

Adjacent sequences: A118388 A118389 A118390 this_sequence A118392 A118393 A118394

easy,frac,nonn

Jonathan Vos Post (jvospost2(AT)yahoo.com), Apr 27 2006

More terms from Alexander Adamchuk (alex(AT)kolmogorov.com), May 08 2007