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A113472
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If d(n) is the sequence of prime differences prime(n+1) - prime(n), then a(n) is the subsequence of d(n) such that d(n) is a power.
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4
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1, 4, 4, 4, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 8, 4, 8, 4, 8, 4, 8, 4, 4, 8, 4, 8, 4, 4, 4, 4, 8, 8, 8, 4, 8, 4, 8, 4, 4, 4, 4, 4, 4, 4, 8, 8, 4, 4, 8, 4, 4, 4, 8, 8, 8, 4, 4, 4, 4, 8, 4, 4, 8, 4, 4, 4, 8, 4, 8, 4, 8, 4, 4, 4, 4, 4, 8, 4, 8, 16, 4, 4, 16, 8, 4, 4, 8, 4, 16, 4, 8, 4, 8, 16, 4, 8
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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EXAMPLE
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a(90) = prime(296) - prime(295) = 1949 - 1933 = 16 = 2^4.
a(329) = prime(1184) - prime(1183) = 9587 - 9551 = 36 = 6^2 (first term not a power of 2).
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MAPLE
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egcd := proc(n) local L; L:=ifactors(n)[2]; L:=map(proc(z) z[2] end, L); igcd(op(L)) end; M:=[]: cnt:=0: for z to 1 do for k from 1 to 200 do p:=ithprime(k); q:=nextprime(p); x:=q-p; if egcd(x)>1 then cnt:=cnt+1; M:=[op(M), [cnt, k, x]] fi od od; M; map(proc(z) z[3] end, M);
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MATHEMATICA
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f[n_] := GCD @@ Last /@ FactorInteger[n] != 1; Select[Table[Prime[n + 1] - Prime[n], {n, 350}], f] (* Ray Chandler, Oct 19 2006 *)
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CROSSREFS
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KEYWORD
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easy,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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