A134037 Concatenated first and last digits of Mersenne prime reversals.

33, 77, 13, 71, 18, 11, 75, 72, 12, 16, 71, 71, 16, 75, 71, 71, 14, 12, 11, 72, 14, 13, 12, 14, 14, 14, 18, 75, 75, 15, 77, 71, 11, 74, 18, 16, 11, 14, 19, 71, 72, 71, 13, 11, 72, 11, 13

Offset: 1

Author

Enoch Haga, Oct 02 2007

Keywords

• nonn
• base
• more

Comments

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).

Examples

			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.
		

Links

• Carlos Rivera, Puzzle 417. M(e) reversed primes, The Prime Puzzles & Problems Connection.

Crossrefs

Cf. A134038 A134039.

Programs

Mathematica

f[n_] := FromDigits[Part[IntegerDigits[n], {-1, 1}]]; f /@ (2^ MersennePrimeExponent[Range[47]] - 1) (* Amiram Eldar, Feb 16 2020 *)

Formula

Generate the Mersenne prime sequence. Reverse the primes. Find the value of the first and last digits and concatenate.

Extensions

a(21)-a(47) from Amiram Eldar, Feb 16 2020