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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

%I #8 Jun 11 2021 12:23:29

%S 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,

%T 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,

%U 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

%N a(n) is the least emirp k such that 2*n-k is also an emirp, or 0 if there are none.

%H Robert Israel, <a href="/A345191/b345191.txt">Table of n, a(n) for n = 1..10000</a>

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

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

%p L:= convert(n,base,10);

%p end proc:

%p filter:= proc(t) local r;

%p if not isprime(t) then return false fi;

%p r:= revdigs(t);

%p r <> t and isprime(r)

%p end proc:

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

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

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

%p t:= 2*n;

%p j:= ListTools:-BinaryPlace(E,t);

%p i:= 1;

%p while i <= j do

%p w:= E[i]+E[j];

%p if w = t then return E[i]

%p elif w < t then i:=i+1

%p else j:= j-1

%p fi

%p od;

%p 0

%p end proc:

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

%Y Cf. A006567

%K nonn,base,look

%O 1,13

%A _J. M. Bergot_ and _Robert Israel_, Jun 10 2021

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Last modified September 17 18:39 EDT 2024. Contains 375990 sequences. (Running on oeis4.)