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 A060461 Numbers k such that 6*k-1 and 6*k+1 are twin composites. 10
 20, 24, 31, 34, 36, 41, 48, 50, 54, 57, 69, 71, 79, 86, 88, 89, 92, 97, 104, 106, 111, 116, 119, 130, 132, 134, 136, 139, 141, 145, 149, 150, 154, 160, 167, 171, 174, 176, 179, 180, 189, 190, 191, 193, 196, 201, 207, 209, 211, 212, 219, 222, 223, 224, 225, 226 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A counterpart to A002822, which generates twin primes. Intersection of A046953 and A046954. - Michel Marcus, Sep 27 2013 All terms can be expressed as (6ab+a+b OR 6cd-c-d) AND (6xy+x-y) for a,b,c,d,x,y positive integers. Example: 20=6*2*2-2-2 AND 20=6*3*1+3-1. - Pedro Caceres, Apr 21 2019 LINKS Zak Seidov, Table of n, a(n) for n = 1..5000. FORMULA a(n) ~ n. More specifically, a(n) = n + 6n/log n - O(n/log^2 n). - Charles R Greathouse IV, Dec 30 2017 EXAMPLE a(9)=57: the 9th twin composites among the odds are {6*57-1}, {6*57+1}, i.e., (341,343) or (11*31, 7^3). MAPLE iscomp := proc(n) if n=1 or isprime(n) then RETURN(0) else RETURN(1) fi: end: for n from 1 to 500 do if iscomp(6*n-1)=1 and iscomp(6*n+1)=1 then printf(`%d, `, n) fi: od: # James A. Sellers, Apr 11 2001 MATHEMATICA Select[Range[200], !PrimeQ[6#-1]&&!PrimeQ[6#+1]&] (* Vladimir Joseph Stephan Orlovsky, Aug 07 2008 *) Select[Range[300], AllTrue[6#+{1, -1}, CompositeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Feb 15 2015 *) Select[Range@ 300, Times @@ Boole@ Map[CompositeQ, 6 # + {1, -1}] > 0 &] (* Michael De Vlieger, Sep 14 2016 *) PROG (PARI) A060461()={my(maxx=5000); n=1; ctr=0; while(ctr

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Last modified December 7 05:14 EST 2019. Contains 329839 sequences. (Running on oeis4.)