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 A256545 Composite n such that n*phi(n) is in A002378. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Composite n such that 4*A002618(n)+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. n=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, n=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. LINKS Robert Israel, Table of n, a(n) for n = 1..62 EXAMPLE a(1) = 6 is in the sequence because 6*phi(6) = 12 = 4*3. MAPLE select(n -> not isprime(n) and issqr(1+4*n*numtheory:-phi(n)), [\$1..10^6]); MATHEMATICA Select[Range[10^6], !PrimeQ[#]&&IntegerQ[Sqrt[4*#*EulerPhi[#]+1]]&] (* Ivan N. Ianakiev, Apr 02 2015 *) PROG (PARI) lista(nn) = {forcomposite (n=1, nn, if (ispolygonal(n*eulerphi(n)/2, 3), print1(n ", ")); ); } \\ Michel Marcus, Apr 02 2015 CROSSREFS Cf. A001653, A002378, A002618, A080806. Sequence in context: A201135 A111876 A119634 * A075591 A130075 A066388 Adjacent sequences:  A256542 A256543 A256544 * A256546 A256547 A256548 KEYWORD nonn AUTHOR Robert Israel, Apr 01 2015 STATUS approved

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Last modified June 4 04:39 EDT 2020. Contains 334815 sequences. (Running on oeis4.)