A134038 Mersenne indices of Mersenne prime reversals which are odd composites (associated with A134039).

4, 6, 7, 11, 12, 14, 15, 16, 19, 22, 28, 29, 30

Offset: 1

Author

Enoch Haga, Oct 02 2007

Keywords

• nonn
• base
• more

Comments

Suggested by Puzzle 417 of Carlos Rivera's The Prime Puzzles & Problems Connection The program below, slightly modified, was used to find the first prime divisor of the Mersenne reversal if an odd composite

Examples

			The Mersenne index 4 is that of 2^7-1 = 127. The reversal is 721 whose first prime divisor is 7 and 7*103=721.
		

Links

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

Crossrefs

Cf. A000043, A134037, A134039.

Programs

Mathematica

rev[n_] := FromDigits @ Reverse @ IntegerDigits[n]; oddComp[n_] := OddQ[n] && CompositeQ[n]; Select[Range[20], oddComp[rev[2^MersennePrimeExponent[#] - 1]] &] (* Amiram Eldar, Feb 16 2020 *)

UBASIC

05 'for a(4):
10 'primes using counters
20 N=727501488517303786137132964064381141071
30 A=3:S=sqrt(N):C="c"
40 B=N\A
50 if B*A=N then print B;A;N;"-";:N=N+2:goto 30
60 A=A+2
70 if A<=sqrt(N) then 40
80 if N>2 then stop 81 C=C+1
90 print C;N;"-";
100 N=N+2:goto 30

Formula

Generate the Mersenne primes, reverse; if the reversal is an odd composite find the first prime divisor associated with the Mersenne index number

Extensions

a(3) inserted and a(6)-a(13) added by Amiram Eldar, Feb 16 2020