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%I #11 Dec 14 2020 05:14:37
%S 5,7,13,17,31,61,83,53,109,317,149,107,367,211,229,577,293,421,197,
%T 211,359,233,409,971,1063,503,1129,2459,541,1229,631,911,409,2909,743,
%U 1051,1093,811,829,859,1609,1619,571,1733,983,983,2309,2003,6581,683,2557,1193,5051,1249,1279,2887,2957
%N a(n) is the least prime p > prime(n+1) such that p == -prime(n+1) (mod prime(n)).
%C a(n) >= 4*prime(n)-prime(n+1), with equality when prime(n) is in A227907.
%H Robert Israel, <a href="/A339691/b339691.txt">Table of n, a(n) for n = 1..10000</a>
%e For n=5 we have prime(5)=11 and prime(6)=13, and a(5)=31 because of the numbers == -13 (mod 11) and greater than 13 (20, 31, ...), 31 is the first prime.
%p f:= proc(n) local p,q,r0, r;
%p p:= ithprime(n);
%p q:= nextprime(p);
%p for r from 4*p-q by p do if isprime(r) then return r fi od;
%p end proc:
%p map(f, [$1..100]);
%o (PARI) a(n) = my(p=prime(n+2)); while(Mod(p, prime(n)) != -prime(n+1), p = nextprime(p+1)); p; \\ _Michel Marcus_, Dec 13 2020
%Y Cf. A227907.
%K nonn,look
%O 1,1
%A _J. M. Bergot_ and _Robert Israel_, Dec 13 2020
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