|
%I #8 Feb 16 2020 02:41:57
%S 33,77,13,71,18,11,75,72,12,16,71,71,16,75,71,71,14,12,11,72,14,13,12,
%T 14,14,14,18,75,75,15,77,71,11,74,18,16,11,14,19,71,72,71,13,11,72,11,
%U 13
%N Concatenated first and last digits of Mersenne prime reversals.
%C Not all reversals of Mersenne primes are primes. Concatenation is a convenient way to see whether the prime reversal might be prime (obviously not if ending in an even number or 5).
%H Carlos Rivera, <a href="https://www.primepuzzles.net/puzzles/puzz_417.htm">Puzzle 417. M(e) reversed primes</a>, The Prime Puzzles & Problems Connection.
%F Generate the Mersenne prime sequence. Reverse the primes. Find the value of the first and last digits and concatenate.
%e a(4)=71 because the first and last digits of the 4th Mersenne prime 127 are 1 and 7. Reversed they are 7 and 1 and concatenated for convenience, 71.
%t f[n_] := FromDigits[Part[IntegerDigits[n], {-1, 1}]]; f /@ (2^ MersennePrimeExponent[Range[47]] - 1) (* _Amiram Eldar_, Feb 16 2020 *)
%Y Cf. A134038 A134039.
%K nonn,base,more
%O 1,1
%A _Enoch Haga_, Oct 02 2007
%E a(21)-a(47) from _Amiram Eldar_, Feb 16 2020
|
