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 A134038 Mersenne indices of Mersenne prime reversals which are odd composites (associated with A134039). 3
 4, 6, 7, 11, 12, 14, 15, 16, 19, 22, 28, 29, 30 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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 LINKS Carlos Rivera, Puzzle 417. M(e) reversed primes, The Prime Puzzles & Problems Connection. FORMULA Generate the Mersenne primes, reverse; if the reversal is an odd composite find the first prime divisor associated with the Mersenne index number EXAMPLE 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. 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 *) PROG UBASIC 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 CROSSREFS Cf. A000043, A134037, A134039. Sequence in context: A310654 A103057 A039590 * A247546 A110605 A206775 Adjacent sequences:  A134035 A134036 A134037 * A134039 A134040 A134041 KEYWORD nonn,base,more AUTHOR Enoch Haga, Oct 02 2007 EXTENSIONS a(3) inserted and a(6)-a(13) added by Amiram Eldar, Feb 16 2020 STATUS approved

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Last modified November 29 21:32 EST 2021. Contains 349416 sequences. (Running on oeis4.)