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A134037
Concatenated first and last digits of Mersenne prime reversals.
3
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
1,1
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).
LINKS
Carlos Rivera, Puzzle 417. M(e) reversed primes, The Prime Puzzles & Problems Connection.
FORMULA
Generate the Mersenne prime sequence. Reverse the primes. Find the value of the first and last digits and concatenate.
EXAMPLE
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.
MATHEMATICA
f[n_] := FromDigits[Part[IntegerDigits[n], {-1, 1}]]; f /@ (2^ MersennePrimeExponent[Range[47]] - 1) (* Amiram Eldar, Feb 16 2020 *)
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Enoch Haga, Oct 02 2007
EXTENSIONS
a(21)-a(47) from Amiram Eldar, Feb 16 2020
STATUS
approved