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
KEYWORD
nonn
AUTHOR
EXTENSIONS
Offset corrected by Donovan Johnson, Jan 16 2012
STATUS
approved