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 A217836 a(n) is largest semiprime < 2*a(n-1), with a(1) = 4. 1
 4, 6, 10, 15, 26, 51, 95, 187, 371, 737, 1473, 2942, 5878, 11755, 23507, 47013, 94021, 188041, 376069, 752135, 1504261, 3008503, 6017001, 12034001, 24068001, 48135995, 96271987, 192543973, 385087943, 770175883, 1540351763, 3080703523, 6161407045, 12322814089, 24645628171 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is to Bertrand primes A006992 as semiprimes A001358 are to primes A000040. LINKS Robert G. Wilson v, Table of n, a(n) for n = 1...225 EXAMPLE a(2) = 6 because that is the largest semiprime < 2*a(1) = 8, where a(1) is the first semiprime. a(3) = 10, the largest semiprime < 2*6 = 12. MATHEMATICA PrevSemiPrime[n_, k_] := Block[{c = 0, sp = n - 1}, While[c < k, While[ PrimeOmega[sp] != 2, sp--]; sp--; c++]; sp + 1]; NestList[ PrevSemiPrime[ 2#, 1] &, 4, 34] (* Robert G. Wilson v, Oct 19 2012 *) CROSSREFS Cf. A001358, A006992. Sequence in context: A116996 A004399 A247558 * A215232 A186777 A028282 Adjacent sequences:  A217833 A217834 A217835 * A217837 A217838 A217839 KEYWORD nonn,easy AUTHOR Jonathan Vos Post, Oct 19 2012 EXTENSIONS Terms greater than a(15) from Robert G. Wilson v, Oct 19 2012 STATUS approved

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Last modified August 5 06:46 EDT 2020. Contains 336209 sequences. (Running on oeis4.)