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A122118
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Least prime factor of 2^n + 5^n.
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2
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2, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 97, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 193, 7, 29, 7, 73, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 97, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 274568286337, 7, 29, 7, 137, 7, 29, 7, 17, 7, 29, 7, 457
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OFFSET
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0,1
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COMMENTS
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a(n_odd)=7, a(n=2+4k,k=0,1,...)=29, a(64)=274568286337 is unusually large.
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LINKS
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FORMULA
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MATHEMATICA
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Table[FactorInteger[2^n+5^n][[1, 1]], {n, 0, 80}] (* or *) Riffle[Table[ FactorInteger[2^n+5^n][[1, 1]], {n, 0, 80, 2}], 7] (* The second program is faster *) (* Harvey P. Dale, Mar 02 2015 *)
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PROG
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(PARI) A122118(n) = { my(k=(2^n+5^n)); forprime(p=if(64==n, 274568286337, 2), k, if(!(k%p), return(p))); }; \\ Antti Karttunen, Nov 02 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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