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A274242
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Numbers whose factorization contains only the greater members of twin prime pairs.
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1
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1, 5, 7, 13, 19, 25, 31, 35, 43, 49, 61, 65, 73, 91, 95, 103, 109, 125, 133, 139, 151, 155, 169, 175, 181, 193, 199, 215, 217, 229, 241, 245, 247, 271, 283, 301, 305, 313, 325, 343, 349, 361, 365, 403, 421, 427, 433, 455, 463, 475, 511, 515, 523, 545, 559, 571, 589, 601, 619, 625, 637, 643, 661, 665, 695, 721, 755, 763, 775, 793, 811, 817, 823, 829, 845, 859, 875, 883, 905, 931
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OFFSET
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1,2
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COMMENTS
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1 is in the sequence by convention.
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LINKS
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Robert Israel, Table of n, a(n) for n = 1..10000
Benoit Cloitre, Plot of a(n)/A274212(n)
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FORMULA
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Arithmetic conjecture: the equation a(n+1) - a(n) = 2r has infinitely many solutions for any fixed integer value r >= 1.
Analytic conjecture: a(n) is asymptotic to D*n*log(n)^2 for a constant D > 0.4 hence lim_{n->oo} a(n)/A274212(n) exists (see link).
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EXAMPLE
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65 = 5*13 is a term because there are twin prime pairs (3,5) and (11,13).
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MAPLE
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N:= 10^4: # to get all terms <= N
Primes:= select(isprime, {seq(i, i=3..N, 2)}):
Twins:= Primes intersect map(`+`, Primes, 2):
S:= {1}:
for p in Twins do
S:= map(s -> seq(s*p^i, i=0..ilog[p](floor(N/s))), S)
od:
sort(convert(S, list)); # Robert Israel, Jun 15 2016
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MATHEMATICA
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Join[{1}, Select[Range[1, 1000, 2], AllTrue[FactorInteger[#][[All, 1]], PrimeQ[# - 2]& ]& ] ] (* Jean-François Alcover, Jun 16 2016 *)
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PROG
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(PARI) for(n=1, 1000, if(prod(i=1, omega(n), isprime(factor(n)[i, 1]-2))==1, print1(n, ", ")))
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CROSSREFS
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Cf. A006512, A274212.
Sequence in context: A314326 A314327 A215804 * A163427 A215805 A339692
Adjacent sequences: A274239 A274240 A274241 * A274243 A274244 A274245
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KEYWORD
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nonn
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AUTHOR
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Benoit Cloitre, Jun 15 2016
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STATUS
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approved
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