Search: id:A116980
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%I A116980
%S A116980 1,1,0,0,0,0,0,0,1,1,4,19,23,16,43,59,66,127,492,886,964,2595,11426,
%T A116980 36780,78070,131232,423402,1302893
%N A116980 Number of ways to arrange integers 1...n so that the sum of each adjacent 
               pair is a triangular number, not counting reversals.
%e A116980 a(9)=1, since {3,7,8,2,4,6,9,1,5} and its reversal are the only permutations
%e A116980 of 1..9 with the given property. Here 3+7, 7+8, 8+2, 4+6, 6+9, 9+1 and 
               1+5 are all triangular numbers.
%Y A116980 Cf. A051239.
%Y A116980 Sequence in context: A043056 A012879 A072178 this_sequence A022135 A028564 
               A003338
%Y A116980 Adjacent sequences: A116977 A116978 A116979 this_sequence A116981 A116982 
               A116983
%K A116980 nonn
%O A116980 1,11
%A A116980 Giovanni Resta (g.resta(AT)iit.cnr.it), Apr 01 2006

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