A345664 Primes whose digit reversal is the product of two (not necessarily distinct) emirps.

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

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