A123710 - OEIS

A123710

Indices k such that 4 = A123709(k) = number of nonzero terms in row k of triangle A123706.

4, 6, 8, 9, 16, 25, 27, 32, 49, 64, 81, 121, 125, 128, 169, 243, 256, 289, 343, 361, 512, 529, 625, 729, 841, 961, 1024, 1331, 1369, 1681, 1849, 2048, 2187, 2197, 2209, 2401, 2809, 3125, 3481, 3721, 4096, 4489, 4913, 5041, 5329, 6241, 6561, 6859, 6889, 7921, 8192, 9409

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OFFSET

1,1

COMMENTS

Triangle A123706 is the matrix inverse of triangle A010766, where A010766(n,k) = [n/k].

Except for a(2)=6, these are proper prime powers, i.e., numbers p^k where k>1, p prime (A025475). - M. F. Hasler, Feb 12 2012

LINKS

Table of n, a(n) for n=1..52.

FORMULA

a(n) = A025475(n) for n>2 (conjectured). - M. F. Hasler, Feb 12 2012

PROG

(PARI) for(n=1, 1e4, A123709(n)==4 & print1(n", ")) \\ M. F. Hasler, Feb 12 2012