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%I #11 Jan 15 2021 01:58:28
%S 2,3,5,7,22,23,25,27,32,33,35,37,52,53,55,57,72,73,75,77,202,203,205,
%T 207,212,213,215,217,222,223,225,227,232,233,235,237,242,243,245,247,
%U 252,253,255,257,262,263,265,267,272,273,275,277,282,283,285,287,292
%N Numbers such that both the left half of the digits and right half of the digits form a prime.
%C If the number of digits in the number is odd > 1, then the middle digit is ignored.
%F 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.
%e 22 is the first number with this property having more than 1 digit.
%t 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 *)
%o (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",") ) ) }
%Y Cf. A125525.
%K base,easy,nonn
%O 1,1
%A _Cino Hilliard_, Jan 29 2007
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