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 A177212 Numbers k that are the products of two distinct primes such that 2*k-1, 4*k-3 and 8*k-7 are also products of two distinct primes. 10
 247, 249, 295, 395, 422, 478, 493, 502, 519, 589, 634, 694, 721, 755, 955, 1255, 1267, 1294, 1306, 1351, 1387, 1441, 1522, 1546, 1727, 1762, 1942, 2031, 2119, 2155, 2323, 2374, 2449, 2491, 2509, 2533, 2587, 2623, 2661, 2733, 2773, 3005, 3039, 3091, 3334 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE 247 is a term because 247 = 13*19, 2*247 - 1 = 493 = 17*29, 4*247-3 = 985 = 5*197, and 8*247 - 1 = 1969 = 11*179. MAPLE isA006881:= proc(n) local F;   F:= ifactors(n)[2];   nops(F)=2 and F[1, 2]+F[2, 2]=2 end proc: filter:= n -> andmap(isA006881, [n, 2*n-1, 4*n-3, 8*n-7]); select(filter, [\$1..10000]); # Robert Israel, Jul 11 2017 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], AppendTo[lst, n]], {n, 0, 3*7!}]; lst p2dpQ[n_]:=Transpose[FactorInteger[n]][[2]]=={1, 1}; With[{s=Select[Range[ 3500], p2dpQ]}, Select[s, AllTrue[{2#-1, 4#-3, 8#-7}, p2dpQ]&]] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Mar 27 2015 *) CROSSREFS Cf. A006881, A177210, A177211. Sequence in context: A289334 A251516 A208188 * A044983 A175043 A051977 Adjacent sequences:  A177209 A177210 A177211 * A177213 A177214 A177215 KEYWORD nonn AUTHOR Vladimir Joseph Stephan Orlovsky, May 04 2010 STATUS approved

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Last modified April 22 22:19 EDT 2021. Contains 343197 sequences. (Running on oeis4.)