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A256545
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Composite numbers k such that k*phi(k) is in A002378.
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2
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6, 30, 434, 510, 616, 912, 1640, 2989, 3003, 5934, 7280, 8600, 10726, 12700, 13825, 14288, 18699, 19389, 54153, 59394, 59906, 70563, 72816, 116052, 117964, 121954, 131070, 134212, 140752, 177000, 206514, 210728, 274023, 319522, 418610, 437736, 456666
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OFFSET
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1,1
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COMMENTS
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Composite k such that 4*A002618(k)+1 is a square.
For all primes p, 4*A002618(p) + 1 = (2*p-1)^2.
The only semiprime < 10^7 in the sequence is 6.
k = 2*p with p prime is in the sequence if 2*p-1 is in A001653. However, the only such p < 10^3000 is 3.
Similarly, k = 3*p with p prime is in the sequence if 2*p-1 is in A080806. However, the only such p < 10^3000 is 2.
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LINKS
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EXAMPLE
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a(1) = 6 is in the sequence because 6*phi(6) = 12 = 4*3.
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MAPLE
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select(n -> not isprime(n) and issqr(1+4*n*numtheory:-phi(n)), [$1..10^6]);
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MATHEMATICA
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Select[Range[10^6], !PrimeQ[#]&&IntegerQ[Sqrt[4*#*EulerPhi[#]+1]]&] (* Ivan N. Ianakiev, Apr 02 2015 *)
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PROG
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(PARI) lista(nn) = {forcomposite (n=1, nn, if (ispolygonal(n*eulerphi(n)/2, 3), print1(n ", ")); ); } \\ Michel Marcus, Apr 02 2015
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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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