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Search: id:A125663
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| A125663 |
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Numbers such that the left half of the digits form a prime and the right half do not. |
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+0
1
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| 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; listen)
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OFFSET
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1,1
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COMMENT
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If n is odd > 1 then the middle digit is ignored.
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FORMULA
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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.
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EXAMPLE
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20 is the first number with this property.
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PROGRAM
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(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", ") ) ) }
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CROSSREFS
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Cf. A125523.
Sequence in context: A138602 A030605 A063780 this_sequence A138601 A008940 A014368
Adjacent sequences: A125660 A125661 A125662 this_sequence A125664 A125665 A125666
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KEYWORD
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base,easy,nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)hotmail.com), Jan 29 2007
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