A125663 - OEIS

%I #3 Oct 01 2013 21:35:20

%S 20,21,24,26,28,29,30,31,34,36,38,39,50,51,54,56,58,59,70,71,74,76,78,

%T 79,200,201,204,206,208,209,210,211,214,216,218,219,220,221,224,226,

%U 228,229,230,231,234,236,238,239,240,241,244,246,248,249,250,251,254,256

%N Numbers such that the left half of the digits form a prime and the right half do not.

%C If n 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 20 is the first number with this property.

%o (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",") ) ) }

%Y Cf. A125523.

%K base,easy,nonn

%O 1,1

%A _Cino Hilliard_, Jan 29 2007