login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A247838 Numbers n such that sigma(sigma(n)) is prime. 4
3, 2667, 3937, 57337, 172011, 253921, 677207307, 1073602561, 732959441001382539, 750688035198863979, 1000923107604038521, 1108158528150703969 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers n such that A051027(n) is a prime p.

Prime 3 is the only prime p such that sigma(sigma(p)) is a prime q.

Conjecture: Subsequence of A046528 (numbers that are a product of distinct Mersenne primes).

Corresponding values of primes p: 7, 8191, 8191, 131071, 524287, 524287, ... (A247822). Conjecture: values of primes p is equal to Mersenne primes (A000668).

732959441001382539, 750688035198863979, 1000923107604038521, 1108158528150703969 and 196751176038481899983340171 are terms. - Jaroslav Krizek, Mar 25 2015

a(9) > 10^10. - Michel Marcus, Feb 13 2020

a(13) > 10^19. - Giovanni Resta, Feb 14 2020

LINKS

Table of n, a(n) for n=1..12.

FORMULA

a(n) = 2*A247821(n)-1.

EXAMPLE

2667 is a term because sigma(sigma(2667)) = sigma(4096) = 8191 (i.e., prime).

MAPLE

with(numtheory): A247838:=n->`if`(isprime(sigma(sigma(n))), n, NULL): seq(A247838(n), n=1..10^5); # Wesley Ivan Hurt, Oct 02 2014

PROG

(Magma) [n: n in [1..10000000] | IsPrime(SumOfDivisors(SumOfDivisors(n)))]

(PARI) isok(n) = isprime(sigma(sigma(n))); \\ Michel Marcus, Oct 01 2014

CROSSREFS

Cf. A000203, A023194, A063103, A000668, A046528, A051027, A247821, A247822, A247954.

Sequence in context: A171361 A203687 A034316 * A003534 A202520 A281928

Adjacent sequences: A247835 A247836 A247837 * A247839 A247840 A247841

KEYWORD

nonn,more

AUTHOR

Jaroslav Krizek, Sep 28 2014

EXTENSIONS

a(7)-a(8) from Michel Marcus, Oct 02 2014

a(9)-a(12) from Giovanni Resta, Feb 14 2020

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 27 18:55 EST 2023. Contains 359845 sequences. (Running on oeis4.)