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 A085245 Least k such that k*2^n + 1 is a semiprime. 2
 4, 2, 1, 2, 1, 1, 1, 6, 3, 2, 1, 1, 1, 6, 3, 2, 1, 2, 1, 1, 3, 2, 1, 3, 8, 4, 2, 1, 3, 2, 1, 1, 3, 7, 5, 5, 8, 4, 2, 1, 4, 2, 1, 3, 3, 7, 6, 3, 15, 9, 29, 28, 14, 7, 6, 3, 3, 8, 4, 2, 1, 4, 2, 1, 14, 7, 12, 6, 3, 3, 9, 5, 12, 6, 3, 8, 4, 2, 1, 3, 29, 18, 9, 18, 9, 10, 5, 13, 8, 4, 2, 1, 15, 12, 6, 3, 9, 6 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The first few values of n such that 78557*2^n + 1 is a semiprime, where k = 78557 (the conjectured smallest Sierpinski number), are: 2, 3, 7, 15, 17, 18, 24, 60, 71, 89, 92, 107, 140, 143, 163,... Conjecture: there are infinitely many semiprimes of this form. LINKS Sean A. Irvine, Table of n, a(n) for n = 1..500 EXAMPLE a(51)=29 because k*2^51 + 1 is not a semiprime for k=1,2,...28, but 29*2^51 + 1 = 63839 * 1022920073887 is. PROG (PARI) a(n) = my(k=1); while (bigomega(k*2^n + 1) != 2, k++); k; \\ Michel Marcus, Jul 02 2020 CROSSREFS Cf. A001358, A035050, A076336. Sequence in context: A010313 A159823 A075826 * A327694 A348136 A235795 Adjacent sequences: A085242 A085243 A085244 * A085246 A085247 A085248 KEYWORD nonn AUTHOR Jason Earls, Aug 11 2003 STATUS approved

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Last modified December 6 01:37 EST 2023. Contains 367594 sequences. (Running on oeis4.)