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%I #18 Oct 02 2017 11:57:37
%S 33,69,295,423,451,531,581,1119,2893,4043,4163,4257,4509,5975,12977,
%T 14927,15753,19105,25369,27225,38013,38799,39771,85675,98531,157375,
%U 217349,264439,324643,330977,352593,382881,417661
%N Odd composite n such that phi(n) * sigma(n) is one less than a square.
%H Donovan Johnson, <a href="/A015722/b015722.txt">Table of n, a(n) for n = 1..300</a>
%H Richard K. Guy, <a href="http://www.jstor.org/stable/2974586">Divisors and desires</a>, Amer. Math. Monthly, 104 (1997), 359-360.
%t 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 *)
%o (PARI) is(n)=n%2&&!isprime(n)&&issquare(eulerphi(n=factor(n))*sigma(n)+1) \\ _Charles R Greathouse IV_, Dec 09 2013
%K nonn
%O 1,1
%A _Robert G. Wilson v_
%E Offset corrected by _Donovan Johnson_, Jan 16 2012
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