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A059843 a(n) is the smallest prime p such that p-n is a nonzero square. 4
2, 3, 7, 5, 41, 7, 11, 17, 13, 11, 47, 13, 17, 23, 19, 17, 53, 19, 23, 29, 37, 23, 59, 73, 29, 107, 31, 29, 173, 31, 47, 41, 37, 43, 71, 37, 41, 47, 43, 41, 617, 43, 47, 53, 61, 47, 83, 73, 53, 59, 67, 53, 89, 79, 59, 137, 61, 59, 383, 61, 97, 71, 67, 73, 101, 67, 71, 149, 73 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Zak Seidov, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = min{p : p - n = x^2 for some x > 0, p is prime}.

Does a(n) exist for all n? - Jianing Song, Feb 04 2019

EXAMPLE

For n = 17, let P = {2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,...} be the set of primes, then P - 17 = {-15,...,-4,0,2,6,12,14,20,24,26,30,36,...}. The first positive square in P - 17 is 36 with p = 53, so a(17) = 53. The square arising here is usually 1.

MAPLE

SearchLimit := 100;

for n from 1 to 400 do

k := 0: c := true:

while(c and k < SearchLimit) do

    k := k + 1:

    c := not isprime(k^2+n):

end do:

if k = SearchLimit then error("Search limit reached!") fi;

a[n] := k^2 + n end do: seq(a[j], j=1..400);

# Edited and SearchLimit introduced by Peter Luschny, Feb 05 2019

MATHEMATICA

spsq[n_]:=Module[{p=NextPrime[n]}, While[!IntegerQ[Sqrt[p-n]], p= NextPrime[ p]]; p]; Array[spsq, 70] (* Harvey P. Dale, Nov 10 2017 *)

PROG

(PARI) for(n=1, 100, for(k=1, 100, if(isprime(k^2+n), print1(k^2+n, ", "); break()))) \\ Jianing Song, Feb 04 2019

(PARI) a(n) = forprime(p=n, , if ((p-n) && issquare(p-n), return (p))); \\ Michel Marcus, Feb 05 2019

CROSSREFS

These terms arise in A002496, A056899, A049423, A005473, A056905, A056909 as first or 2nd entries depending on offset.

Cf. A002496, A056899, A049423, A005473, A056905, A056909.

Cf. A056896 (where p-n can be 0).

Sequence in context: A258126 A332211 A069587 * A092927 A071553 A021812

Adjacent sequences:  A059840 A059841 A059842 * A059844 A059845 A059846

KEYWORD

nonn

AUTHOR

Labos Elemer, Feb 26 2001

STATUS

approved

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Last modified June 12 19:42 EDT 2021. Contains 344961 sequences. (Running on oeis4.)