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%I #12 Jan 26 2021 14:25:48
%S 21,33,55,77,87,91,111,115,119,129,155,161,185,215,235,247,249,259,
%T 267,287,291,295,301,303,305,323,339,341,355,361,365,381,417,427,453,
%U 469,481,485,501,505,511,517,527,533,537,551,573,589,591
%N Semiprimes s such that r=(s^2+1)/2 is also a semiprime.
%C Semiprimes which are a leg of an integral right triangle whose hypotenuse is also semiprime. This is to A048161 as semiprimes A001358 are to primes A000040. All terms must be odd (else r is not an integer).
%F {s such that s = p_1 * q_1 for p_1, q_1 primes, and r=(s^2+1)/2 = p_2 * q_2 for p_2, q_2 primes}.
%e a(1) = 21 because 21 = 3*7 is semiprime, and (21^2+1)/2 = 221 = 13 * 17 is semiprime.
%e a(2) = 33 because 33 = 3 * 11 is semiprime, and (33^2+1)/2 = 545 = 5 * 109 is semiprime.
%e a(3) = 55 because 55 = 5 * 11 is semiprime, and (55^2+1)/2 = 1513 = 17 * 89 is semiprime.
%o (PARI) is_A176945(n)={ bittest(n,0) & bigomega(n)==2 & bigomega(1+n^2\2)==2 } \\ _M. F. Hasler_, Dec 08 2010
%Y Cf. A001358, A048161.
%K nonn,easy
%O 1,1
%A _Jonathan Vos Post_, Dec 08 2010
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