OFFSET
2,2
LINKS
Matthew Conroy, Table of n, a(n) for n = 2..10000
Charles Small, Waring's problem mod n, Amer. Math. Monthly 84 (1977), no. 1, 12--25.
FORMULA
From Small's paper, theorem 3.1: a(n)=1 if n=2; else a(n)=2 if n != 0 mod 4 and p^2|n implies p=1 mod 4; else a(n)=3 if n!=0 mod 8; else a(n)=4.
EXAMPLE
0^2 = 0 and 1^2 = 1 mod 2, so each element of Z/2Z is a square, so a(2)=1;
0^2 = 0, 1^2 = 2^2 = 1 mod 3, so 2 = 1^2 + 1^2 requires two squares to sum to 2, so a(3)=2.
MATHEMATICA
a[n_] := Which[n == 2, 1, Mod[n, 4] != 0 && AllTrue[Select[Divisors[n] // Sqrt, IntegerQ], Mod[#, 4] == 1&], 2, Mod[n, 8] != 0, 3, True, 4];
Table[a[n], {n, 2, 140}] (* Jean-François Alcover, Jun 13 2017 *)
PROG
(PARI) c(n) = A=factor(n); ok=1; for(i=1, matsize(A)[1], if(A[i, 1]%4==3&&A[i, 2]>1, ok=0)); return(ok); wn(n) = if(n==2, 1, if(n%4>0&&c(n)==1, 2, if(n%8>0, 3, 4))); for(ii=2, 140, print1(wn(ii), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Matthew Conroy, Jun 13 2017
STATUS
approved