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 A129910 Quotient of the decimal representation of concatenated twin primes in reverse divided by 3. 2
 17, 25, 437, 639, 1043, 1447, 2053, 2457, 34367, 36369, 46379, 50383, 60393, 64397, 66399, 76409, 80413, 90423, 94427, 104437, 116449, 140473, 144477, 154487, 174507, 190523, 200533, 206539, 214547, 220553, 270603, 274607, 276609, 286619 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Except for the first term, concatenated twin primes reversed are always divisible by 3. This follows from the fact that twin prime components > 3 in reverse 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 reverse is 53. The quotient of 53/3 is 17 which is the first term. MATHEMATICA qdr[{a_, b_}]:=Quotient[FromDigits[Flatten[IntegerDigits/@{b, a}]], 3]; qdr/@ Select[Partition[Prime[Range], 2, 1], #[]-#[]==2&] (* Harvey P. Dale, Mar 02 2018 *) PROG (PARI) concattwins3r(n) = { local(x, y); forprime(x=2, n, if(isprime(x+2), y=floor(eval(concat(Str(x+2), Str(x)))/3); print1(y", ") ) ) } CROSSREFS Sequence in context: A183346 A166666 A147445 * A259075 A212909 A273785 Adjacent sequences:  A129907 A129908 A129909 * A129911 A129912 A129913 KEYWORD base,frac,nonn AUTHOR Cino Hilliard, Jun 05 2007 STATUS approved

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Last modified September 17 08:54 EDT 2019. Contains 327127 sequences. (Running on oeis4.)