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

Author

Cino Hilliard, Jan 29 2007

Keywords

• base
• easy
• nonn

Comments

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

Examples

			20 is the first number with this property.
		

Crossrefs

Cf. A125523.

Programs

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",") ) ) }

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.