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 A177213 Numbers k that are the products of two distinct primes such that 2*k-1, 4*k-3, 8*k-7 and 16*k-15 are also products of two distinct primes. 9
 247, 295, 478, 634, 694, 721, 1255, 1267, 1294, 1387, 1546, 1762, 1942, 2323, 2374, 2773, 3005, 3334, 3403, 3883, 3949, 4126, 4714, 4741, 4777, 5062, 5269, 5287, 5353, 5422, 5617, 6583, 6805, 7273, 7495, 8587, 8767, 9017, 9406, 9427, 9847, 10018 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE 247 is a term because 247 = 13*19, 2*247 - 1 = 493 = 17*29, 4*247 - 3 = 985 = 5*197, 8*247 - 1 = 1969 = 11*179, and 16*247 - 15 = 3937 = 31*127. MATHEMATICA f[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; Do[If[f[n]&&f[2*n-1]&&f[4*n-3]&&f[8*n-7]&&f[16*n-15], AppendTo[lst, n]], {n, 0, 8!}]; lst ptdpQ[n_]:=PrimeNu[n]==PrimeOmega[n]==2; Select[Range[11000], AllTrue[ {#, 2#-1, 4#-3, 8#-7, 16#-15}, ptdpQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 30 2016 *) CROSSREFS Cf. A006881, A177210, A177211, A177212. Sequence in context: A044983 A175043 A051977 * A283737 A020313 A318055 Adjacent sequences:  A177210 A177211 A177212 * A177214 A177215 A177216 KEYWORD nonn AUTHOR Vladimir Joseph Stephan Orlovsky, May 04 2010 STATUS approved

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Last modified December 8 17:01 EST 2021. Contains 349596 sequences. (Running on oeis4.)