Table of n, a(n) for n=1..5.
28 is member because the number of proper divisors of 28 is 5, a prime number.
Cf. A000396, A032741, A133033, A154896.
Sequence in context: A038182 A095723 A057246 * A276493 A074849 A189373
Adjacent sequences: A154892 A154893 A154894 * A154896 A154897 A154898
Omar E. Pol, Jan 25 2009