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%I #8 Dec 25 2021 09:32:06
%S 1,5,21,23,25,33,81,85,115,127,141,164,253,273,283,285,291,343,385,
%T 441,471,495,505,565,577,711,807,921,1107,1977,2175,2437,2941,2943,
%U 3381,4117,5541,6531,7075,7497,8193,8325,8923
%N Numbers k such that pi(k) = Sum_{i=1..k} pi(k*(i-1)+i) - pi(k*(i-1)+i-1).
%C Numbers with the same number of primes on the top row and along the main diagonal of an n X n square array whose elements are the numbers from 1..n^2, listed in increasing order by rows (see example).
%F Numbers k such that A000720(k) = A221490(k).
%e 5 is in the sequence since there are 3 primes in the top row and 3 primes along the main diagonal of the 5 X 5 array below.
%e [1 2 3 4 5]
%e [6 7 8 9 10]
%e [11 12 13 14 15]
%e [16 17 18 19 20]
%e [21 22 23 24 25]
%Y Cf. A000720 (pi), A221490.
%K nonn
%O 1,2
%A _Wesley Ivan Hurt_, Dec 24 2021