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 A225774 Primes p such that sigma(sigma(p)) = 2*phi(p); sigma(n) = A000203(n); phi(n) = A000010(n). 0

%I #13 Aug 01 2013 18:27:51

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

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

%C There are no other terms <= 2*10^8.

%C Also primes p such that sigma(p + 1) = 2*phi(p) = 2p - 2.

%C 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.

%C Subsequence of A135241 (numbers n such that sigma(sigma(n)) = 2*phi(n).

%C Primes p such that sigma(p+1)-2*(p+1) = -4. - _Donovan Johnson_, Aug 01 2013

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

%Y Cf. A000203, A000010, A135241, A125246.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Jul 26 2013

%E a(7) from _Donovan Johnson_, Aug 01 2013

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Last modified September 14 06:54 EDT 2024. Contains 375920 sequences. (Running on oeis4.)