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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)
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
Sequence in context: A310654 A103057 A039590 * A247546 A110605 A206775
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 July 13 02:50 EDT 2024. Contains 374265 sequences. (Running on oeis4.)