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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 14 23:14 EDT 2024. Contains 375171 sequences. (Running on oeis4.)