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A061782 Smallest number such that n*a(n) is a triangular number. 3
1, 3, 1, 7, 2, 1, 3, 15, 4, 1, 5, 3, 6, 2, 1, 31, 8, 2, 9, 6, 1, 3, 11, 5, 12, 3, 13, 1, 14, 4, 15, 63, 2, 4, 3, 1, 18, 5, 2, 3, 20, 5, 21, 12, 1, 6, 23, 11, 24, 6, 3, 15, 26, 7, 1, 21, 3, 7, 29, 2, 30, 8, 6, 127, 5, 1, 33, 2, 4, 3, 35, 28, 36, 9, 4, 21, 3, 1, 39, 26, 40, 10, 41, 14, 7, 11, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

FORMULA

For p an odd prime, a(p) = (p-1)/2. For nonnegative k, a(2^k) = 2^(k+1)-1.

Formula corrected by Nick Singer, Jun 26 2006

EXAMPLE

a(4) = 7 as 4*7 = 28 is a triangular number and 7 is the smallest such number.

MAPLE

isA000217 := proc(n)

    issqr(1+8*n) ;

end proc:

A016782 := proc(n)

    local a;

    for a from 1 do

        if isA000217(n*a) then

            return a;

        end if;

    end do:

end proc:

seq(A016782(n), n=1..40) ; # R. J. Mathar, Oct 03 2014

MATHEMATICA

snt[n_]:=Module[{k=1}, While[!OddQ[Sqrt[1+8k n]], k++]; k]; Array[snt, 100] (* Harvey P. Dale, Feb 15 2017 *)

CROSSREFS

Sequence in context: A146265 A146217 A146007 * A074625 A284877 A169659

Adjacent sequences:  A061779 A061780 A061781 * A061783 A061784 A061785

KEYWORD

nonn,easy

AUTHOR

Amarnath Murthy, May 24 2001

EXTENSIONS

Corrected and extended by Matthew Conroy, May 28 2001

STATUS

approved

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Last modified August 3 20:08 EDT 2020. Contains 336201 sequences. (Running on oeis4.)