This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A186621 #9 Mar 10 2015 12:59:21 %S A186621 3,5,8,9,13,14,20,21,24,25,32,33,34,37,38,45,48,50,54,56,57,61,64,68, %T A186621 73,76,81,84,85,86,90,92,93,94,105,110,114,117,118,120,121,122,128, %U A186621 132,133,140,141,142,144,145,154,157,158,160,165,168,176,177,182,184,186,193 %N A186621 Semiprimes - 1. %C A186621 Numbers of the form k-1 where k is semiprime (or biprime), namely a product of exactly two (not necessarily distinct) primes. Used as relative base sequence for analogies to sequences involving number of form (p-1) for prime p. %H A186621 Robert Price, <a href="/A186621/b186621.txt">Table of n, a(n) for n = 1..1366</a> %F A186621 a(n) = A001358(n) - 1. %e A186621 The smallest semiprime is 4 = 2*2, so a(1) = 4-1 = 3. %t A186621 Select[Range[5000], 2 == Total[Transpose[FactorInteger[# + 1]][[2]]] &] %Y A186621 Cf. A001358, A006093 Primes minus 1. %K A186621 nonn,easy %O A186621 1,1 %A A186621 _Jonathan Vos Post_, Feb 24 2011