

A225774


Primes p such that sigma(sigma(p)) = 2*phi(p); sigma(n) = A000203(n); phi(n) = A000010(n).


0




OFFSET

1,1


COMMENTS

There are no other terms <= 2*10^8.
Also primes p such that sigma(p + 1) = 2*phi(p) = 2p  2.
Also primes p such that sigma(sigma(p))  sigma(p)  p = 3. The only composite number <= 2*10^8 with this property is the number 4.
Subsequence of A135241 (numbers n such that sigma(sigma(n)) = 2*phi(n).


LINKS



EXAMPLE

sigma(sigma(13)) = 2*phi(13) = 24.


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



