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A345191 a(n) is the least emirp k such that 2*n-k is also an emirp, or 0 if there are none. 1
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 13, 0, 13, 0, 17, 0, 0, 0, 0, 13, 0, 17, 13, 0, 17, 0, 0, 0, 31, 0, 0, 31, 0, 0, 37, 0, 0, 0, 0, 13, 13, 17, 17, 13, 0, 17, 0, 0, 31, 31, 0, 37, 13, 0, 17, 37, 0, 13, 0, 17, 13, 31, 17, 0, 37, 0, 31, 0, 71, 31, 73, 0, 37, 73, 0, 0, 79, 0, 13, 0, 17, 71, 13, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,13

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

a(24) = 17 because 2*24 = 17+31 where 17 and 31 are emirps.

MAPLE

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

  L:= convert(n, base, 10);

  add(L[-i]*10^(i-1), i=1..nops(L))

end proc:

filter:= proc(t) local r;

if not isprime(t) then return false fi;

r:= revdigs(t);

r <> t and isprime(r)

end proc:

N:= 1000: # for a(1)..a(N)

E:= select(filter, [seq(i, i=3..2*N, 2)]);

f:= proc(n) local i, j, w, t;

  t:= 2*n;

  j:= ListTools:-BinaryPlace(E, t);

  i:= 1;

  while i <= j do

    w:= E[i]+E[j];

    if w = t then return E[i]

    elif w < t then i:=i+1

    else j:= j-1

    fi

  od;

  0

end proc:

map(f, [$1..N]);

CROSSREFS

Cf. A006567

Sequence in context: A221341 A221106 A271075 * A073867 A114782 A065112

Adjacent sequences:  A345188 A345189 A345190 * A345192 A345193 A345194

KEYWORD

nonn,base,look

AUTHOR

J. M. Bergot and Robert Israel, Jun 10 2021

STATUS

approved

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Last modified August 11 12:12 EDT 2022. Contains 356065 sequences. (Running on oeis4.)