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A345664 Primes whose digit reversal is the product of two (not necessarily distinct) emirps. 0
1187, 1291, 1621, 1931, 3119, 3637, 3833, 3989, 7069, 7129, 7411, 9049, 9181, 9461, 9631, 10103, 10589, 10597, 10691, 12517, 12653, 12703, 13487, 13999, 14771, 14969, 15319, 15359, 16183, 16253, 16411, 16421, 16477, 16823, 16901, 17027, 17881, 18061, 18121, 19079, 19121, 19427, 19507, 19739 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..44.

EXAMPLE

a(3) = 1621 is a term because 1621 is prime and its digit reversal is 1261 = 13*97 where 13 and 97 are emirps.

MAPLE

digrev:= proc(n) local L, i;

  L:= convert(n, base, 10);

  add(L[-i]*10^(i-1), i=1..nops(L))

end proc:

isemirp:= proc(p) local r;

  if not isprime(p) then return false fi;

  r:= digrev(p);

  r <> p and isprime(r)

end proc:

E:= select(isemirp, [seq(seq(seq(i*10^d+j, j=1..10^d-1, 2), i=[1, 3, 7, 9]), d=1..3)]):

R:= NULL:

for i from 1 to nops(E) do

for j from 1 to i do

    v:= E[i]*E[j];

    if v > 10^5 then break fi;

    v:= digrev(v);

    if isprime(v) then R:= R, v fi

od od:

sort([R]);

CROSSREFS

Cf. A006567.

Sequence in context: A345540 A345792 A237698 * A052236 A342066 A153378

Adjacent sequences:  A345661 A345662 A345663 * A345665 A345666 A345667

KEYWORD

nonn,base

AUTHOR

J. M. Bergot and Robert Israel, Jun 21 2021

STATUS

approved

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Last modified May 26 00:47 EDT 2022. Contains 354073 sequences. (Running on oeis4.)