A103741 a(n) is a non-palindromic composite located between twin primes whose reverse, which is less than it, is also located between twin primes.

60, 240, 270, 600, 810, 822, 2130, 2340, 2802, 8010, 8220, 8430, 8838, 8862, 20550, 22740, 23202, 23370, 23910, 25410, 26880, 27240, 28410, 28572, 28662, 29022, 29760, 80472, 81702, 81930, 81972, 82140, 82530, 83220, 83340, 83640, 85620

Offset

1,1

Links

Table of n, a(n) for n=1..37.

Example

810 is in the sequence as it is between the twin primes 809 and 811 and its reverse, 018 or 18, is also between the twin primes 17 and 19.

Mathematica

fQ[n_] := Block[{np = FromDigits[ Reverse[ IntegerDigits[n]]]}, If[np < n && PrimeQ[n - 1] && PrimeQ[n + 1] && PrimeQ[np - 1] && PrimeQ[np + 1], True, False]]; Select[ Range[2, 87220, 2], fQ[ # ] &] (* Robert G. Wilson v, Mar 30 2005 *)
npcQ[n_]:=Module[{ridn=FromDigits[Reverse[IntegerDigits[n]]]}, ridn<n && And@@PrimeQ[ {ridn-1, ridn+1}]]; Select[Mean/@Select[Partition[ Prime[ Range[ 2, 8500]], 2, 1], Last[#]-First[#]==2&], npcQ] (* Harvey P. Dale, Oct 19 2012 *)

Crossrefs

Sequence in context: A256985 A075287 A277990 * A140873 A263225 A019285

Adjacent sequences: A103738 A103739 A103740 * A103742 A103743 A103744

Keyword

base,nonn

Author

Ray G. Opao, Mar 28 2005

Extensions

Corrected and extended by Robert G. Wilson v, Mar 30 2005

Status

approved