A358859 - OEIS

%I #11 Dec 06 2022 09:51:11

%S 6,36,210,4560,6426,326040,4232250,1969110,296676380,4798080,

%T 166289760,73765692000,712750500,50561280,33944067893736,

%U 2139168754800,4292572951800,1414764341760,72461756727360,180975331456920,1870768457500800,5498331930000,153698278734000

%N a(n) is the smallest n-gonal number divisible by exactly n n-gonal numbers.

%C The corresponding indices of n-gonal numbers are 3, 6, 12, 48, 51, 330, 1100, 702, ...

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PolygonalNumber.html">Polygonal Number</a>

%H <a href="/index/Di#divisors">Index entries for sequences related to divisors of numbers</a>

%e a(5) = 210, because 210 is a pentagonal number that has 5 pentagonal divisors {1, 5, 35, 70, 210} and this is the smallest such number.

%o (PARI) a(n) = if(n<3, return()); for(k=1, oo, my(t=(k*(n*k - n - 2*k + 4))\2); if(sumdiv(t, d, ispolygonal(d, n)) == n, return(t))); \\ _Daniel Suteu_, Dec 04 2022

%Y Cf. A005179, A076983, A358539, A358860, A358861.

%K nonn

%O 3,1

%A _Ilya Gutkovskiy_, Dec 03 2022

%E a(11)-a(25) from _Daniel Suteu_, Dec 04 2022