A130317 - OEIS

A130317

Smallest number having exactly n triangular divisors.

1, 3, 6, 36, 30, 90, 180, 210, 420, 630, 1890, 1260, 2520, 6930, 18480, 20790, 13860, 27720, 41580, 83160, 138600, 245700, 235620, 180180, 556920, 360360, 540540, 1670760, 1081080, 1413720, 2702700, 2162160, 6486480, 3063060, 8288280

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OFFSET

1,2

COMMENTS

2*a(n) is smallest number having exactly n oblong divisors.

A007862(a(n)) = n and A007862(m) <> n for m < a(n).

LINKS

Ray Chandler and Donovan Johnson, Table of n, a(n) for n = 1..100 (first 58 terms from Ray Chandler)

FORMULA

a(n) = A088726(n-1)/2 for n>1. - Ray Chandler, Jun 24 2008

EXAMPLE

a(3)=6: A007862(6)=#{1,2*(2+1)/2,3*(3+1)/2}=3;

a(4)=36: A007862(36)=#{1,2*(2+1)/2,3*(3+1)/2,8*(8+1)/2}=4;

PROG

(PARI) a(n) = my(k=1); while (sumdiv(k, d, ispolygonal(d, 3)) != n, k++); k; \\ Michel Marcus, Jan 14 2022

CROSSREFS

Cf. A007862, A088726, A130279.

Sequence in context: A359423 A358765 A358669 * A019467 A106128 A119186

Adjacent sequences: A130314 A130315 A130316 * A130318 A130319 A130320

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, May 23 2007

EXTENSIONS

Extended by Ray Chandler, Jun 24 2008

STATUS

approved