A223457 Composite numbers whose number of proper divisors has a number of proper divisors which has a prime number of proper divisors.
44100, 46656, 57600, 65536, 108900, 112896, 152100, 213444, 260100, 278784, 298116, 313600, 324900, 331776, 389376, 476100, 509796, 592900, 636804, 656100, 665856, 736164, 756900, 774400, 828100, 831744, 864900, 933156, 1000000, 1081600, 1218816, 1232100
Offset: 1
Keywords
Examples
a(1) = 44100, which has 80 divisors. 80 has 9 divisors. 9 has 2 divisors, 2 is prime. 3 steps were needed.
Programs
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Mathematica
d3Q[n_]:=PrimeQ[Nest[DivisorSigma[0,#]-1&,n,3]]; Select[Range[13*10^5],d3Q] (* Harvey P. Dale, Apr 21 2016 *)