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!)
A109197 Minimal value of k > 0 such that n^2 + k^2 is semiprime. 10
2, 3, 9, 1, 3, 1, 7, 3, 1, 1, 11, 1, 1, 3, 3, 1, 3, 3, 11, 1, 9, 2, 1, 2, 11, 1, 3, 4, 1, 1, 1, 2, 7, 5, 1, 1, 7, 4, 5, 1, 7, 2, 1, 2, 1, 1, 1, 3, 1, 1, 1, 1, 1, 2, 5, 2, 5, 4, 1, 1, 1, 1, 1, 2, 1, 1, 5, 7, 3, 1, 9, 1, 11, 4, 3, 2, 1, 2, 1, 1, 1, 14, 5, 2, 5, 1, 1, 5, 1, 6, 7, 2, 1, 2, 7, 1, 1, 6, 13, 8, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..100.

FORMULA

a(n) = minimal value of k > 0 such that n^2 + k^2 is semiprime.

EXAMPLE

a(0) = 2 because 0^2 + 1^2 = 1 is not semiprime, but 0^2 + 2^2 = 4 = 2^2 is.

a(1) = 3 because 1^2 + 1^2 and 1^2 + 2^2 are not semiprime, but 1^2 + 3^2 = 10 = 2 * 5 is semiprime.

a(81) = 14 because 81^2 + 14^2 = 6757 = 29 * 233 and for no smaller k>0 is 81^2 + k^2 a semiprime.

a(100) = 1 because 100^2 + 1^2 = 10001 = 73 * 137.

MATHEMATICA

k2sp[n_]:=Module[{k=1}, While[PrimeOmega[n^2+k^2]!=2, k++]; k]; Array[ k2sp, 110, 0] (* Harvey P. Dale, Oct 30 2016 *)

PROG

(PARI) A109197(n)={local(r); r=1; while(bigomega(n^2+r^2)<>2, r=r+1); r} \\ Michael B. Porter, May 13 2010

CROSSREFS

Cf. A001358, A108714.

Sequence in context: A011163 A155983 A201407 * A021811 A280987 A030367

Adjacent sequences:  A109194 A109195 A109196 * A109198 A109199 A109200

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, Jun 21 2005

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 September 24 03:26 EDT 2021. Contains 347623 sequences. (Running on oeis4.)