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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 March 28 15:02 EDT 2020. Contains 333089 sequences. (Running on oeis4.)