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A225774
Primes p such that sigma(sigma(p)) = 2*phi(p); sigma(n) = A000203(n); phi(n) = A000010(n).
0
13, 43, 109, 151, 883, 2143, 34360131583
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 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
EXAMPLE
sigma(sigma(13)) = 2*phi(13) = 24.
CROSSREFS
KEYWORD
nonn,more,changed
AUTHOR
Jaroslav Krizek, Jul 26 2013
EXTENSIONS
a(7) from Donovan Johnson, Aug 01 2013
STATUS
approved