A253474 Numbers n such that the difference between n^2 and largest prime less than n^2 is not prime.

2, 11, 17, 23, 25, 31, 39, 41, 45, 51, 53, 56, 57, 59, 65, 67, 73, 76, 79, 81, 83, 85, 87, 91, 95, 97, 99, 100, 101, 105, 109, 111, 113, 115, 123, 125, 129, 133, 137, 141, 143, 147, 149, 151, 153, 154, 157, 159, 163, 165, 167, 170, 171, 175, 179, 181, 185, 187, 189, 193, 195, 197, 199, 201, 203, 207, 209, 213, 215, 219, 221, 225

Offset: 1

Author

Carlos Eduardo Olivieri, Mar 17 2015

Keywords

• nonn
• easy

Comments

Indices of terms in A056927 that are not prime.

Examples

			a(1) = 2, since 2^2 - 3 = 1.
a(2) = 11, since 11^2 - 113 = 8.
a(3) = 17, since 17^2 - 283 = 6.
a(4) = 23, since 23^2 - 523 = 6.
		

Links

• Richard L. Francis, Between consecutive squares, Missouri Journal of Mathematical Sciences, Volume 16 Issue #1 - Winter 2004.

Crossrefs

Cf. A000040, A000290, A056927, A053001.

Programs

Mathematica

f[n_] := n^2 - NextPrime[n^2, -1]; Select[Range[2, 230], !PrimeQ[f[#]] &]

PARI

lista(nn) = for (n=2, nn, if (!isprime(n^2-precprime(n^2)), print1(n, ", "))); \\ Michel Marcus, Mar 22 2015