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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

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: A138602 A030605 A063780 * A241850 A138601 A218539

Adjacent sequences:  A125660 A125661 A125662 * A125664 A125665 A125666

KEYWORD

base,easy,nonn

AUTHOR

Cino Hilliard, Jan 29 2007

STATUS

approved

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Last modified August 28 08:35 EDT 2015. Contains 261118 sequences.