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!)
A193881 Numbers n such that 10^n-sigma(n^2) is prime. 2
7, 52, 446, 614, 1137, 4852, 5615, 22154, 71291 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

sigma(x) is even unless x is a square or twice a square, therefore 10^n-sigma(n) can't be prime unless n is a square or twice a square, and {2, 49} are the only solutions < 10^4.

a(10) > 10^5. - Robert Price, Mar 24 2015

LINKS

Table of n, a(n) for n=1..9.

MATHEMATICA

Select[Range[1000], PrimeQ[10^# - DivisorSigma[1, #^2]] &] (* Robert Price, Mar 24 2015 *)

PROG

(PARI) for(n=1, 9999, ispseudoprime(t=10^n-sigma(n^2)) && print1(n", "))

(Magma) [n: n in [1..450] | IsPrime(10^n-DivisorSigma(1, n^2))]; // Vincenzo Librandi, Mar 26 2015

CROSSREFS

Cf. A110065, A110066, A110067, A173837, A174176.

Sequence in context: A349532 A162233 A185623 * A249349 A300492 A179517

Adjacent sequences:  A193878 A193879 A193880 * A193882 A193883 A193884

KEYWORD

nonn,hard

AUTHOR

M. F. Hasler, Aug 07 2011

EXTENSIONS

a(6)-a(9) from Robert Price, Mar 25 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 3 09:13 EDT 2022. Contains 357231 sequences. (Running on oeis4.)