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A125665
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Numbers such that both the left half of the digits and right half of the digits form a prime.
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0
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2, 3, 5, 7, 22, 23, 25, 27, 32, 33, 35, 37, 52, 53, 55, 57, 72, 73, 75, 77, 202, 203, 205, 207, 212, 213, 215, 217, 222, 223, 225, 227, 232, 233, 235, 237, 242, 243, 245, 247, 252, 253, 255, 257, 262, 263, 265, 267, 272, 273, 275, 277, 282, 283, 285, 287, 292
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OFFSET
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1,1
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COMMENTS
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If the number of digits in the number is odd > 1, then the middle digit is ignored.
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LINKS
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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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22 is the first number with this property having more than 1 digit.
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MATHEMATICA
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lhrhQ[n_]:=Module[{idn=IntegerDigits[n], len=Floor[IntegerLength[n]/2]}, And @@ PrimeQ[FromDigits/@{Take[idn, len], Take[idn, -len]}]]; Join[ {2, 3, 5, 7}, Select[Range[300], lhrhQ]] (* Harvey P. Dale, Jul 05 2013 *)
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PROG
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(PARI) bothprime(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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KEYWORD
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base,easy,nonn
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AUTHOR
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STATUS
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approved
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