login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A265425 Numbers n such that n+2 and sigma(n-1) are both primes. 0
3, 5, 17, 65, 4097, 65537, 262145, 1073741825 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If a(9) exists, it must be larger than A023194(10000) = 5896704025969.

Prime terms: 3, 5, 17, 65537, ...

Any prime present must be one of the lesser twin primes (A001359) and also a Fermat prime (A019434), at least. See comments in A023194. - Antti Karttunen, Dec 08 2015

Sequence is different from A256438; numbers 1152921504606846977, 309485009821345068724781057, 81129638414606681695789005144065 and 85070591730234615865843651857942052865 are not terms of this sequence.

Numbers 2^m+1 such that 2^m + 3 and 2^(m+1) - 1 are both prime. - Hiroaki Yamanouchi, Jan 04 2016

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

Number 17 is in the sequence because 17 + 2 = 19 and sigma(17-1) = sigma(16) = 31; 17 and 31 are primes.

MATHEMATICA

Select[Range[10^7], And[PrimeQ[# + 2], PrimeQ[DivisorSigma[1, # - 1]]] &] (* Michael De Vlieger, Dec 09 2015 *)

PROG

(MAGMA) [n: n in [2..1000000] | IsPrime(n+2) and IsPrime(SumOfDivisors(n-1))]

(PARI) for(n=2, 10^7, if(ispseudoprime(n+2) && ispseudoprime(sigma(n-1)), print1(n, ", "))) \\ Altug Alkan, Dec 08 2015

CROSSREFS

Cf. A000203, A001359, A019434, A023194, A256438.

Sequence in context: A085749 A281623 A278741 * A256438 A251737 A125957

Adjacent sequences:  A265422 A265423 A265424 * A265426 A265427 A265428

KEYWORD

nonn,more

AUTHOR

Jaroslav Krizek, Dec 08 2015

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 25 03:58 EDT 2019. Contains 324338 sequences. (Running on oeis4.)