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%I #17 Jan 25 2021 15:26:22
%S 4,6,8,9,16,25,27,32,49,64,81,121,125,128,169,243,256,289,343,361,512,
%T 529,625,729,841,961,1024
%N Indices n such that 4 = A123709(n) = number of nonzero terms in row n of triangle A123706.
%C Triangle A123706 is the matrix inverse of triangle A010766, where A010766(n,k) = [n/k].
%C 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
%F a(n) = A025475(n) for n>2 (conjectured). - _M. F. Hasler_, Feb 12 2012
%o (PARI) for(n=1,1e4, A123709(n)==4 & print1(n",")) \\ _M. F. Hasler_, Feb 12 2012
%Y Cf. A123706, A123709, A123711, A123712; A010766.
%K nonn,more
%O 1,1
%A _Paul D. Hanna_, Oct 09 2006