A342550 - OEIS

%I #64 Mar 29 2021 09:31:13

%S 2,2,2,5,2,2,5,6,2,5,2,2,10,6,2,5,2,2,11,6,2,5,7,2,5,13,2,5,2,2,5,6,7,

%T 19,2,2,5,6,2,5,2,2,19,6,2,5,9,2,11,6,2,5,17,2,5,6,2,5,2,2,5,14,7,22,

%U 2,2,5,13,2,5,2,2,10,6,2,17,2,2,20,6,2,5,7,2,5

%N For n>=3, a(n) is the sum of the indices of n seen as an m-gonal number.

%C This sum is constituted of A177025(n) terms related to the n-row of A177028 triangle.

%C For m in A090467, a(m) = 2.

%C By definition, a(n) can never be equal to 3 or 4.

%C Up to 10^7, no n has been found with a(n) = 8, 12 or 18.

%H Michel Marcus, <a href="/A342550/b342550.txt">Table of n, a(n) for n = 3..10000</a>

%e 15 is the 5th triangular, the 3rd hexagonal and the 2nd 15-gonal, so a(15) = 5+3+2 = 10.

%o (PARI) row(n) = my(v=List()); fordiv(2*n, k, if(k<2, next); if(k==n, break); my(s=(2*n/k-4+2*k)/(k-1)); if(denominator(s)==1, listput(v, s))); Vecrev(v); \\ A177028

%o a(n) = my(v=row(n), s=0); for (k=1, #v, if ((v[k]>2) && ispolygonal(n, v[k], &i), s += i)); s;

%Y Cf. A090467, A177025, A177028, A342772.

%K nonn

%O 3,1

%A _Michel Marcus_, Mar 27 2021