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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 (list; graph; refs; listen; history; text; internal format)
 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). Primes p such that sigma(p+1)-2*(p+1) = -4. - Donovan Johnson, Aug 01 2013 LINKS Table of n, a(n) for n=1..7. EXAMPLE sigma(sigma(13)) = 2*phi(13) = 24. CROSSREFS Cf. A000203, A000010, A135241, A125246. Sequence in context: A023262 A067260 A135241 * A268256 A243894 A303740 Adjacent sequences: A225771 A225772 A225773 * A225775 A225776 A225777 KEYWORD nonn AUTHOR Jaroslav Krizek, Jul 26 2013 EXTENSIONS a(7) from Donovan Johnson, Aug 01 2013 STATUS approved

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Last modified August 13 14:39 EDT 2024. Contains 375142 sequences. (Running on oeis4.)