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 A176945 Semiprimes s such that r=(s^2+1)/2 is also a semiprime. 0
 21, 33, 55, 77, 87, 91, 111, 115, 119, 129, 155, 161, 185, 215, 235, 247, 249, 259, 267, 287, 291, 295, 301, 303, 305, 323, 339, 341, 355, 361, 365, 381, 417, 427, 453, 469, 481, 485, 501, 505, 511, 517, 527, 533, 537, 551, 573, 589, 591 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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). LINKS FORMULA {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}. EXAMPLE a(1) = 21 because 21 = 3*7 is semiprime, and (21^2+1)/2 = 221 = 13 * 17 is semiprime. a(2) = 33 because 33 = 3 * 11 is semiprime, and (33^2+1)/2 = 545 = 5 * 109 is semiprime. a(3) = 55 because 55 = 5 * 11 is semiprime, and (55^2+1)/2 = 1513 = 17 * 89 is semiprime. PROG (PARI) is_A176945(n)={ bittest(n, 0) & bigomega(n)==2 & bigomega(1+n^2\2)==2 }  \\ M. F. Hasler, Dec 08 2010 CROSSREFS Cf. A001358, A048161. Sequence in context: A026068 A217263 A330441 * A333857 A070006 A189986 Adjacent sequences:  A176942 A176943 A176944 * A176946 A176947 A176948 KEYWORD nonn,easy AUTHOR Jonathan Vos Post, Dec 08 2010 STATUS approved

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Last modified September 25 12:38 EDT 2021. Contains 347654 sequences. (Running on oeis4.)