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

1,1

Comments

Indices of terms in A056927 that are not prime.

Links

Table of n, a(n) for n=1..72.

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

Example

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.

Mathematica

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

Prog

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

Crossrefs

Cf. A000040, A000290, A056927, A053001.

Sequence in context: A309499 A173638 A018420 * A133410 A156829 A255609

Adjacent sequences: A253471 A253472 A253473 * A253475 A253476 A253477

Keyword

nonn,easy

Author

Carlos Eduardo Olivieri, Mar 17 2015

Status

approved