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A060461
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Numbers k such that 6*k-1 and 6*k+1 are twin composites.
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12
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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
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OFFSET
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1,1
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COMMENTS
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A counterpart to A002822, which generates twin primes.
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
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LINKS
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FORMULA
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a(n) ~ n. More specifically, there are x - x/log x + O(x/log^2 x) terms of the sequence up to x. - Charles R Greathouse IV, Mar 03 2020
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EXAMPLE
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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).
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MAPLE
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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
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MATHEMATICA
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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 *)
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PROG
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(PARI) A060461()={my(maxx=5000); n=1; ctr=0; while(ctr<maxx, if(!isprime(6*n-1)&&!isprime(6*n+1), print1(n, ", "); ctr+=1); n+=1); } \\ Bill McEachen, Apr 04 2015
(MATLAB)
i=1:10000;
Q1 = 6*i-1;
Q2 = 6*i+1;
Q = union(Q1, Q2);
P = primes(max(Q));
AT = setxor(Q, P);
f = 0;
for j=1:numel(AT);
K = AT(j);
K2 = K+2;
z = ismember(K2, AT);
if z == 1;
f = f+1;
ATR(f, :) = K + 1;
end
end
m6 = ATR./6;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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