

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

Table of n, a(n) for n=1..13.
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 2^71 = 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 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



