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 A081974 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. 2
 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, 41, 81, 40, 52, 103, 51, 25, 49, 24, 47, 93, 46, 91, 60, 76, 151, 75, 37, 18, 112, 115, 57, 28, 55, 39, 19, 87, 43, 22, 133, 66, 58, 117, 35, 17, 8, 395, 126, 141, 70, 102, 90, 116, 231 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Perhaps another re-arrangement of natural numbers. LINKS EXAMPLE 5 and 4 are the neighbors of 9 giving the triangular numbers 45 and 36 respectively. PROG (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) CROSSREFS Cf. A081975, A081976, A081977. Sequence in context: A160106 A243786 A278743 * A257883 A169755 A169748 Adjacent sequences:  A081971 A081972 A081973 * A081975 A081976 A081977 KEYWORD nonn AUTHOR Amarnath Murthy, Apr 03 2003 EXTENSIONS More terms from David Wasserman, Jul 26 2004 STATUS approved

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Last modified January 22 02:46 EST 2022. Contains 350481 sequences. (Running on oeis4.)