cp's OEIS Frontend

A078178 Least k>=2 such that n^k + n - 1 is prime.

%I A078178 #10 Aug 18 2025 06:22:27
%S A078178 2,2,2,2,2,3,2,2,2,2,4,2,16,2,2,4,3,2,2,2,7,4,2,3,2,3,2,10,2,2,108,3,
%T A078178 6,2,3,7,2,2,4,2,16,3,2,2,2,20,2,7,2,3,3,2,2,2,2,9,4,2,2,7,8,3,2,2,2,
%U A078178 24,2,6,2,12,4,3,8,6,2,4,3,9,194,3,13,2,8,2,2,3,8,2,10,6,4,2,2,54,2,132,4,10,2
%N A078178 Least k>=2 such that n^k + n - 1 is prime.
%C A078178 n^a(n) + n - 1 = A078179(n).
%e A078178 7^2+7-1=5*11, but 7^3+7-1=349=A000040(70), therefore a(7)=3.
%t A078178 lkp[n_]:=Module[{k=2},While[!PrimeQ[n^k+n-1],k++];k]; Array[lkp,100,2] (* _Harvey P. Dale_, May 24 2020 *)
%o A078178 (Haskell)
%o A078178 a078178 n = head [k | k <- [2..], a010051'' (n ^ k + n - 1) == 1]
%o A078178 -- _Reinhard Zumkeller_, Jul 16 2014
%Y A078178 Cf. A076845, A078179.
%Y A078178 Cf. A010051.
%K A078178 nonn
%O A078178 2,1
%A A078178 _Reinhard Zumkeller_, Nov 20 2002
%E A078178 More terms from _Benoit Cloitre_, Nov 20 2002