A202338 - OEIS

%I #14 Feb 28 2020 18:03:51

%S 1,3,1,1,3,1,1,3,377,1,3,13,1,89,1,3,3,1,1,89,1,3,3,1,1,3,1,1,3,89,1,

%T 21,1,13,3,13,3,3,89,1,233,1,1,3,13,1,21,13,1,3,13,1,233,1,21,3,1,13,

%U 3,1,3,21,1,1,3,1,3,3,1,1,21,13,3,3,1,1,3,233

%N Smallest Fibonacci number which when appended to n produces a prime number.

%H Michel Lagneau, <a href="/A202338/a202338.txt">Table of n, a(n) for n = 1..5000</a>

%e a(9) = 377 because 9 concatenated with 377 gives the prime 9377.

%p with(combinat,fibonacci):for n from 1 to 80 do: i:=0:for m from 1 to 2000 while(i=0) do:p:=fibonacci(m): nn:=length(p): x:=n*10^nn+p: if type(x,prime)=true then i:=1: printf(`%d, `,p):else fi:od:od:

%t f[n_]:=Block[{p=1,a=IntegerDigits[n]},While[!PrimeQ[FromDigits[Join[a,IntegerDigits[Fibonacci[p]]]]],p++];Fibonacci[p]];Table[f[n],{n,92}]

%t With[{fibs=Fibonacci[Range[20]],nn=80},Table[SelectFirst[fibs,PrimeQ[ n*10^ IntegerLength[ #]+ #]&],{n,nn}]] (* _Harvey P. Dale_, Feb 28 2020 *)

%Y Cf. A000045.

%K nonn,base

%O 1,2

%A _Michel Lagneau_, Dec 17 2011