A051697 Closest prime to n (break ties by taking the smaller prime).

2, 2, 2, 3, 3, 5, 5, 7, 7, 7, 11, 11, 11, 13, 13, 13, 17, 17, 17, 19, 19, 19, 23, 23, 23, 23, 23, 29, 29, 29, 29, 31, 31, 31, 31, 37, 37, 37, 37, 37, 41, 41, 41, 43, 43, 43, 47, 47, 47, 47, 47, 53, 53, 53, 53, 53, 53, 59, 59, 59, 59, 61, 61, 61, 61, 67, 67, 67, 67, 67, 71, 71

Offset: 0

Author

N. J. A. Sloane

Keywords

• nonn
• easy

Examples

			Closest primes to 0,1,2,3,4 are 2,2,2,3,3.
		

Links

• Eric Weisstein's World of Mathematics, Nearest Prime

Crossrefs

Related sequences: A023186-A023188, A046929-A046931, A051650, A051652, A051697-A051702, A051728-A051730.

Programs

Mathematica

a[n_] := (np = NextPrime[n]; pp = Prime[PrimePi[np] - 1]; Which[np > 2n-pp, pp, np < 2n-pp, np, True, pp]); a[0] = a[1] = 2; Table[a[n], {n, 0, 71}] (* Jean-François Alcover, Jul 28 2011 *)

PARI

a(n)=if(n<3, return(2)); my(p=precprime(n),q=nextprime(n)); if(q-nCharles R Greathouse IV, Apr 28 2015

Extensions

More terms from James Sellers