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A003140
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Number of coprime chains with largest member prime(n).
(Formerly M0896)
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2
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1, 1, 2, 3, 9, 10, 19, 20, 32, 84, 85, 161, 212, 214, 260, 521, 818, 820, 1189, 1406, 1415, 2005, 2375, 3351, 5698, 6122, 6141, 6600, 6623, 7270, 23993, 26735, 33686, 33753, 55735, 55750, 66498, 85117, 90310, 147374, 165450, 165479, 249822, 250176
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OFFSET
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1,3
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COMMENTS
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A coprime chain is a nonempty set of integers greater than 1 such that all primes <= the largest member divide exactly one term of the set. - Charles R Greathouse IV, Apr 24 2013
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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PROG
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(PARI) isCC(v)=forprime(p=2, vecmax(v), if(sum(i=1, #v, v[i]%p==0)!=1, return(0))); 1
A003139(n)=my(v=vector(n-1, i, i+1)); sum(i=2^(n-2), 2^(n-1)-1, isCC(vecextract(v, i)))
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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