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 A255770 #14 Aug 01 2016 11:09:17 %S A255770 1,2,3,4,6,8,9,11,13,17,19,21 %N A255770 Number of distinct prime factors of A220161(n). %C A255770 This strictly increasing sequence proves (yet again) the infinitude of primes. %D A255770 Arthur Engel, Problem-Solving Strategies, Springer, 1998, pages 121-122 (E3, said to be a "recent competition problem from the former USSR"). %H A255770 Alexander Bogomolny, <a href="http://www.cut-the-knot.org/proofs/InfinitudeOfPrimesViaPowersOfTwo.shtml">Infinitude of Primes via Powers of 2</a> %H A255770 factordb, <a href="http://factordb.com/index.php?query=2%5E%282%5En%29+%2B+2%5E%282%5E%28n+-+1%29%29+%2B+1">First 20 factorizations of A220161</a> %e A255770 A220161(0) = 7 so a(0) = 1. %e A255770 A220161(1) = 3*7 so a(1) = 2. %e A255770 A220161(2) = 3*7*13 so a(2) = 3. %e A255770 A220161(3) = 3*7*13*241 so a(3) = 4. %e A255770 A220161(4) = 3*7*13*97*241*673 so a(4) = 6. %Y A255770 Cf. A220161, A220294, A255771, A255772. %K A255770 nonn,hard,more %O A255770 0,2 %A A255770 _Hans Havermann_, Mar 06 2015 %E A255770 Offset changed by _Arkadiusz Wesolowski_, Aug 01 2016