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A347167
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Numbers k such that phi(binomial(k,2)) is a power of 2.
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0
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2, 3, 4, 5, 6, 16, 17, 256, 257, 65536, 65537, 4294967296
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OFFSET
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1,1
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COMMENTS
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Every Fermat prime appears in this sequence.
A number greater than 2^32 is in this sequence if and only if it is a Fermat prime.
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REFERENCES
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M. Krizek, F. Luca and L. Somer, 17 Lectures on Fermat Numbers: From Number Theory to Geometry, CMS Books in Mathematics, vol. 9, Springer-Verlag, New York, 2001, p. 86.
F. Luca, Pascal's triangle and constructible polygons, Util. Math. 58 (2000d), pp. 209-214.
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LINKS
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FORMULA
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For n >= 13, a(n) = A019434(n-7) (if it exists).
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MATHEMATICA
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PROG
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(Magma) r:=7; IsInteger:=func<i | i eq Floor(i)>; lst:=[k: k in [2..6] | IsInteger(Log(2, EulerPhi(Binomial(k, 2))))]; t:=1; for x in [1..r] do m:=4^(2^x); if t eq 1 then Append(~lst, m); end if; if IsPrime(m+1) then Append(~lst, m+1); else t:=0; end if; end for; lst;
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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STATUS
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approved
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