A129538 - OEIS

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A129538

a(n) = smallest positive integer such that lcm(a(1), a(2), ..., a(n)) is a multiple of the n-th triangular number n(n+1)/2.

1, 3, 2, 5, 1, 7, 4, 9, 1, 11, 1, 13, 1, 1, 8, 17, 1, 19, 1, 1, 1, 23, 1, 25, 1, 27, 1, 29, 1, 31, 16, 1, 1, 1, 1, 37, 1, 1, 1, 41, 1, 43, 1, 1, 1, 47, 1, 49, 1, 1, 1, 53, 1, 1, 1, 1, 1, 59, 1, 61, 1, 1, 32, 1, 1, 67, 1, 1, 1, 71, 1, 73, 1, 1, 1, 1, 1, 79, 1, 81, 1, 83, 1, 1, 1, 1, 1, 89, 1, 1, 1, 1

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..92.

EXAMPLE

The 7th triangular number is 28; lcm(a(1),a(2),a(3),a(4),a(5),a(6),a(7)) = lcm(1,3,2,5,1,7,4) = 420, which is a multiple of 28. 4 is the smallest value a(7) can take and still have the LCM of the first 7 terms of the sequence be a multiple of 28.

MATHEMATICA

a = {1}; l = 1; For[n = 2, n < 80, n++, i = 1; l1 = l; While[ ! Mod[l1, n*(n + 1)/2] == 0, i++; l1 = LCM[l, i]]; AppendTo[a, i]; l = LCM[l, i]]; a (* Stefan Steinerberger, Nov 21 2007 *)

CROSSREFS

Sequence in context: A113260 A051543 A265574 * A076934 A111701 A093527

Adjacent sequences: A129535 A129536 A129537 * A129539 A129540 A129541

KEYWORD

nonn

AUTHOR

Leroy Quet, May 29 2007

EXTENSIONS

More terms from Stefan Steinerberger, Nov 21 2007

STATUS

approved