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%I #22 Jul 04 2019 11:31:11
%S 3,7,3,1,8,1,5,2,2,6,1,1,6,5,1,1,4,2,1,2,4,3,2,4,4,4,8,5,5,5,7,1,1,4,
%T 8,6,1,4,9,1,2,1,3,1,2,1,3
%N Initial digit of Mersenne primes A000668.
%H Landon Curt Noll, <a href="http://www.isthe.com/chongo/tech/math/prime/mersenne.html">Mersenne Prime Digits and Names</a>.
%F a(n) = A000030(A000668(n)). - _Omar E. Pol_, Jul 04 2019
%e a(4)=1 because the 4th Mersenne prime A000668(4) is 127 and the initial digit of 127 is 1.
%t lst = {* the list of terms in A000043 *}; f[n_] := Block[{pn = 2^n - 1}, Quotient[pn, 10^Floor[ Log[10, pn]] ]]; f@# & /@ lst (* _Robert G. Wilson v_, Apr 01 2008 *)
%t IntegerDigits[#][[1]]&/@(2^#-1&/@MersennePrimeExponent[Range[47]]) (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jul 04 2019 *)
%Y Cf. A000030, A000668, A077648, A080172.
%K nonn,base,more
%O 1,1
%A _Omar E. Pol_, Mar 01 2008
%E More terms from _Robert G. Wilson v_, Apr 01 2008
%E a(40)-a(44) from _David Radcliffe_, Jan 21 2016
%E a(45)-a(47) from _Ivan Panchenko_, Apr 11 2018
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