login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A246907 Numbers n such that sigma(n + sigma(n)) = 3n. 1
1, 2, 4, 8, 16, 64, 128, 2048, 262144, 17179869184, 274877906944, 8796093022208, 36028797018963968 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjecture: for n >= 2; numbers n of the form 2^k such that 3*(2^k) - 1 is prime. The next terms: 18446744073709551616, 75557863725914323419136, 19807040628566084398385987584, … Sequence of numbers k: 1, 2, 3, 4, 6, 7, 11, 18, 34, 38, 43, 55, 64, 76, … Subsequence of A087370 (numbers n such that 3n - 1 is a prime).

a(14) > 5*10^17. - Hiroaki Yamanouchi, Sep 11 2015

LINKS

Table of n, a(n) for n=1..13.

EXAMPLE

Number 16 is in sequence because sigma(16 + sigma(16)) = sigma(16 + 31) = sigma(47) = 48 = 3 * 16.

MATHEMATICA

Select[Range[300000], DivisorSigma[1, # + DivisorSigma[1, #]] == 3 # &] (* Harvey P. Dale, Jul 19 2015 *)

PROG

(MAGMA) [n:n in[1..10000000] | SumOfDivisors(n+SumOfDivisors(n))eq 3*n]

(PARI)

for(n=1, 10^7, if(sigma(n+sigma(n))==3*n, print1(n, ", "))) \\ Derek Orr, Sep 07 2014

CROSSREFS

Cf. A000203, A087370, A246906.

Sequence in context: A264980 A318151 A076086 * A074700 A230863 A322037

Adjacent sequences:  A246904 A246905 A246906 * A246908 A246909 A246910

KEYWORD

nonn,more

AUTHOR

Jaroslav Krizek, Sep 07 2014

EXTENSIONS

a(10)-a(13) from Hiroaki Yamanouchi, Sep 11 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 December 6 04:20 EST 2021. Contains 349562 sequences. (Running on oeis4.)