OFFSET
1,1
COMMENTS
Except for the first term, concatenated twin primes are always divisible by 3. This follows from the fact that twin prime components > 3 are of the form 6k-1 and 6k+1. So concatenation in decimal is (6k-1)*10^d + 6k+1 = 6k(10^d+1)+(10^d-1) where d is the number of digits in each twin prime component. Now 10^d-1 = (10-1)(10^(d-1)+10^(d-2)+...+1) = 9h and 6k(10^d+1) + 9h is divided by 3.
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
The first concatenated twin prime pair in decimal representation is 35.
The quotient of 35/3 is 11 which is the first term.
MATHEMATICA
Join[{11}, FromDigits[Flatten[IntegerDigits/@#]]/3&/@Rest[Select[ Partition[ Prime[ Range[200]], 2, 1], Last[#]-First[#]==2&]]] (* Harvey P. Dale, Oct 12 2012 *)
PROG
(PARI) concattwins3(n) = { local(x, y); forprime(x=2, n, if(isprime(x+2), y=eval(concat(Str(x), Str(x+2)))/3; print1(y", ")) ) }
CROSSREFS
KEYWORD
base,frac,nonn
AUTHOR
Cino Hilliard, Jun 05 2007
STATUS
approved