

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
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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



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 2^71 = 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)
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


CROSSREFS



KEYWORD

nonn,base,more


AUTHOR



EXTENSIONS

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


STATUS

approved



