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 A171074 #5 May 01 2013 21:12:46 %S A171074 1,4,18,68,250,922,3430,12868,48618,184754,705430,2704154,10400598, %T A171074 40116598,155117518,601080388,2333606218,9075135298,35345263798, %U A171074 137846528818,538257874438,2104098963718,8233430727598,32247603683098 %N A171074 A115112 with initial term changed from 0 to 1. %C A171074 From a variant of the tennis ball problem (cf. A031970, A049235). On turn n ball 2n-1 is introduced to the room, ball 2n is introduced to the garden, then one of the balls in the room is swapped with one of the balls in the garden. The present sequence gives the number of combinations, while A171075 gives the total on the lawn, A170076 gives the total in the room. %D A171074 David Scambler, Just for fun, more tennis balls, Posting to the Sequence Fans Mailing List, Aug 25 2010. %Y A171074 Cf. A031970, A049235, A171075, A171076. %K A171074 nonn %O A171074 1,2 %A A171074 _N. J. A. Sloane_, Sep 06 2010