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 A015722 Odd composite n such that phi(n) * sigma(n) is one less than a square. 3
 33, 69, 295, 423, 451, 531, 581, 1119, 2893, 4043, 4163, 4257, 4509, 5975, 12977, 14927, 15753, 19105, 25369, 27225, 38013, 38799, 39771, 85675, 98531, 157375, 217349, 264439, 324643, 330977, 352593, 382881, 417661 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Donovan Johnson, Table of n, a(n) for n = 1..300 Richard K. Guy, Divisors and desires, Amer. Math. Monthly, 104 (1997), 359-360. MATHEMATICA nn=420000; okQ[n_]:=IntegerQ[Sqrt[EulerPhi[n]DivisorSigma[1, n]+1]]; Select[Complement[Range[1, nn, 2], Prime[Range[PrimePi[nn]]]], okQ] (* Harvey P. Dale, Mar 27 2011 *) PROG (PARI) is(n)=n%2&&!isprime(n)&&issquare(eulerphi(n=factor(n))*sigma(n)+1) \\ Charles R Greathouse IV, Dec 09 2013 CROSSREFS Sequence in context: A256022 A044135 A044516 * A103046 A063868 A184417 Adjacent sequences:  A015719 A015720 A015721 * A015723 A015724 A015725 KEYWORD nonn AUTHOR EXTENSIONS Offset corrected by Donovan Johnson, Jan 16 2012 STATUS approved

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Last modified April 10 15:05 EDT 2021. Contains 342845 sequences. (Running on oeis4.)