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%I #23 Apr 28 2020 03:12:20
%S 1,1,0,0,1,0,0,0,0,1,0,0,2,0,0,0,1,0,0,0,0,0,0,0,2,1,0,0,5,0,0,0,0,5,
%T 0,0,1,0,0,0,2,0,0,0,0,0,0,0,0,1,0,0,10,0,0,0,0,5,0,0,2,0,0,0,1,0,0,0,
%U 0,0,0,0,10,13,0,0,0,0,0,0,0,1,0,0,2,0,0,0,13,0,0,0,0,0,0,0,5,0,0,0,1,0
%N Smallest k such that nk-1 is a square, or 0 if no such number exists.
%H Reinhard Zumkeller, <a href="/A076948/b076948.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) != 0 if and only if n is a term of A008784. - _Joerg Arndt_, Apr 27 2020
%F a(n) = 1 if and only if n is a term of A002522. - _Bernard Schott_, Apr 27 2020
%t a[n_] := Module[{r, j, k}, r = Solve[j>0 && k>0 && n k - 1 == j^2, {j, k}, Integers]; If[r === {}, Return[0], Return[k /. (r /. C[1] -> 0) // Min]]]; a[1] = 1;
%t Array[a, 100] (* _Jean-François Alcover_, Apr 27 2020 *)
%o (Haskell)
%o a076948 1 = 1
%o a076948 n = if null qs then 0 else head qs
%o where qs = filter ((> 0) . a037213 . subtract 1 . (* n)) [1..n]
%o -- _Reinhard Zumkeller_, Oct 25 2015
%o (PARI) a(n) = if (!issquare(Mod(-1, n)), 0, my(k=1); while (!issquare(n*k-1), k++); k); \\ _Michel Marcus_, Apr 27 2020
%Y Cf. A008784.
%Y Cf. A037213.
%K nonn
%O 1,13
%A _Amarnath Murthy_, Oct 20 2002
%E Edited and extended by _Robert G. Wilson v_, Oct 21 2002
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