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A186265
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a(n) = b_f(n) where f is the 2-periodic sequence f(k) = (-1)^k (see comments).
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1
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2, 5, 11, 23, 41, 61, 107, 197, 311, 617, 1229, 2381, 4649, 8861, 17027, 33809, 67409, 134681, 267719, 535349, 1069217, 2138399, 4275641, 8545697, 17091377, 34182749, 68365469, 136730639, 273461159, 546917141, 1093813727, 2187610991, 4375221077, 8750432231
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OFFSET
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1,1
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COMMENTS
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Let u(1)=1 and u(n)=abs(u(n-1)-gcd(u(n-1),n+f(n)) where f(n) is a periodic sequence with period [f(1),f(2),...,f(beta)]. Then (b_f(k))_{k>=1} is the sequence of integers such that u(b_f(k))=0. We conjecture that for k large enough b_f(k)+1+f(i) is simultaneously prime for i=1,2,...,beta. Here for f(k)=(-1)^k it appears a(n) and a(n)+2 are twin primes for n>=7. If we start with u(1) large enough (such as with u(1)=71) the sequence will produce only twin primes.
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LINKS
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FORMULA
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Conjecture: a(n) is asymptotic to c*2^n with c>0.
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PROG
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(PARI) a=1; for(n=2, 10^9, a=abs(a-gcd(a, n+(-1)^n)); if(a==0, print1(n, ", ")))
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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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