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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A135243 Numbers n such that sigma(n)=6*(reversal(n)-n). 1
34, 394, 3994, 39994, 2309882, 3070463, 7796129, 205207782, 225073882, 478854106, 644154038, 1556396391, 3154222593, 3165974493, 5951924107, 10444787521, 12114130241, 20611191282, 58403366517, 71103822309 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n=4*10^m-6 is a semiprime (and 2*10^m-3 is prime) then n is in the sequence (the proof is easy). Next term is greater than 7*10^7.

LINKS

Table of n, a(n) for n=1..20.

EXAMPLE

sigma(34)=54=6*(43-34)=6*(reversal(34)-34), so 34 is in the sequence.

MATHEMATICA

Do[If[6(FromDigits@Reverse@IntegerDigits@n-n)==DivisorSigma[1, n], Print[n]], {n, 70000000}]

CROSSREFS

Cf. A135242.

Sequence in context: A168122 A271756 A228995 * A283227 A033914 A189452

Adjacent sequences:  A135240 A135241 A135242 * A135244 A135245 A135246

KEYWORD

base,nonn

AUTHOR

Farideh Firoozbakht, Jan 04 2008

EXTENSIONS

a(8)-a(20) from Donovan Johnson, Dec 21 2008

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 August 2 08:47 EDT 2021. Contains 346422 sequences. (Running on oeis4.)