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a(n) = smallest prime such that a(n)+2 is multiple of 2n+1.
1

%I #7 Jul 03 2021 10:57:36

%S 7,3,5,7,31,11,13,83,17,19,67,23,79,317,29,31,103,109,37,367,41,43,

%T 139,47,151,157,53,283,293,59,61,193,199,67,211,71,73,229,709,79,911,

%U 83,433,443,89,277,283,677,97,503,101,103,2459,107,109,337,113,349,593,1087

%N a(n) = smallest prime such that a(n)+2 is multiple of 2n+1.

%C Terms appearing twice: a(1)=a(4)=7, a(5)=a(16)=31,...,

%C terms appearing thrice: a(28)=a(47)=a(142)=283, a(20)=a(61)=a(184)=367, etc.

%H Michael S. Branicky, <a href="/A175452/b175452.txt">Table of n, a(n) for n = 1..10000</a>

%e n=1: 7+2 is multiple of 3, n=2: 3+2 is multiple of 5, n=5: 31+2 is multiple of 11, n=8: 83+2 is multiple of 17.

%t s={};Do[k=2;While[Mod[2+(p=Prime[k]),n]>0,k++ ];AppendTo[s,p],{n,3,2001,2}];s

%o (Python)

%o from sympy import nextprime

%o def a(n):

%o p, m = 2, 2*n+1

%o while (p+2)%m: p = nextprime(p)

%o return p

%o print([a(n) for n in range(1, 61)]) # _Michael S. Branicky_, Jul 03 2021

%o (PARI) a(n) = my(p=2); while ((p+2) % (2*n+1), p = nextprime(p+1)); p; \\ _Michel Marcus_, Jul 03 2021

%Y Cf. A124199.

%K nonn

%O 1,1

%A _Zak Seidov_, May 16 2010