The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A125663 Numbers such that the left half of the digits form a prime and the right half do not. 0
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If n is odd > 1 then the middle digit is ignored. LINKS 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 A030605 A063780 * A241850 A138601 A278582 Adjacent sequences:  A125660 A125661 A125662 * A125664 A125665 A125666 KEYWORD base,easy,nonn AUTHOR Cino Hilliard, Jan 29 2007 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified July 25 16:39 EDT 2021. Contains 346291 sequences. (Running on oeis4.)