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A253474 Numbers n such that the difference between n^2 and largest prime less than n^2 is not prime. 0
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified November 23 21:51 EST 2020. Contains 338603 sequences. (Running on oeis4.)