A225774 Primes p such that sigma(sigma(p)) = 2*phi(p); sigma(n) = A000203(n); phi(n) = A000010(n).

13, 43, 109, 151, 883, 2143, 34360131583

Offset: 1

Author

Jaroslav Krizek, Jul 26 2013

Keywords

• nonn
• more

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 primes in A135241 (numbers k such that sigma(sigma(k)) = 2*phi(k)).

Primes p such that sigma(p+1)-2*(p+1) = -4. - Donovan Johnson, Aug 01 2013

Examples

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

Crossrefs

Cf. A000203, A000010, A135241, A125246.

Extensions

a(7) from Donovan Johnson, Aug 01 2013