|
%I #5 Dec 05 2013 19:56:03
%S 1,3,2,5,9,4,7,13,6,11,21,10,12,23,45,14,27,53,26,30,33,16,31,15,20,
%T 41,81,40,52,103,51,25,49,24,47,93,46,91,60,76,151,75,37,18,112,115,
%U 57,28,55,39,19,87,43,22,133,66,58,117,35,17,8,395,126,141,70,102,90,116,231
%N a(1) = 1 and smallest number not occurring earlier such that the product of two neighboring terms is a distinct triangular number, where "distinct" means that a(n)*a(n+1) may not equal the product of any two previous consecutive terms.
%C Perhaps another re-arrangement of natural numbers.
%e 5 and 4 are the neighbors of 9 giving the triangular numbers 45 and 36 respectively.
%o (PARI) istriang(n) = local(x); x = floor(sqrt(2*n)); n == x*(x + 1)/2; A = vector(75); used = vector(1000); tris = vector(50000); A[1] = 1; used[1] = 1; for (i = 2, 75, f = A[i - 1]; j = 2; while (used[j] || !istriang(f*j) || tris[f*j], j = j + 1); A[i] = j; used[j] = 1; tris[f*j] = 1); print(A)
%Y Cf. A081975, A081976, A081977.
%K nonn
%O 1,2
%A _Amarnath Murthy_, Apr 03 2003
%E More terms from _David Wasserman_, Jul 26 2004
|
