1,1

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).

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

Table of n, a(n) for n=1..7.

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

Cf. A000203, A000010, A135241, A125246.

Sequence in context: A023262 A067260 A135241 * A268256 A243894 A303740

Adjacent sequences: A225771 A225772 A225773 * A225775 A225776 A225777

nonn

Jaroslav Krizek, Jul 26 2013

a(7) from Donovan Johnson, Aug 01 2013

approved