A125663 Numbers such that the left half of the digits form a prime and the right half do not.

20, 21, 24, 26, 28, 29, 30, 31, 34, 36, 38, 39, 50, 51, 54, 56, 58, 59, 70, 71, 74, 76, 78, 79, 200, 201, 204, 206, 208, 209, 210, 211, 214, 216, 218, 219, 220, 221, 224, 226, 228, 229, 230, 231, 234, 236, 238, 239, 240, 241, 244, 246, 248, 249, 250, 251, 254, 256

Offset

1,1

Comments

If n is odd > 1 then the middle digit is ignored.

Links

Table of n, a(n) for n=1..58.

Formula

The left half of an n-digit number is the first floor(n/2) digits. The right half of an n-digit number is the last floor(n/2) digits.

Example

20 is the first number with this property.

Prog

(PARI) leftprime(n) = { local(x, ln, y, lp, rp); for(x=1, n, y=Str(x); if(x > 9, ln=floor(length(y)/2), ln=1); lp = eval(left(y, ln)); rp = eval(right(y, ln)); if(isprime(lp)&& !isprime(rp), print1(x", ") ) ) }

Crossrefs

Cf. A125523.

Sequence in context: A326678 A347469 A030605 * A241850 A138601 A278582

Adjacent sequences: A125660 A125661 A125662 * A125664 A125665 A125666

Keyword

base,easy,nonn

Author

Cino Hilliard, Jan 29 2007

Status

approved