A076409 Sum of the quadratic residues of prime(n).

1, 1, 5, 7, 22, 39, 68, 76, 92, 203, 186, 333, 410, 430, 423, 689, 767, 915, 1072, 994, 1314, 1343, 1577, 1958, 2328, 2525, 2369, 2675, 2943, 3164, 3683, 3930, 4658, 4587, 5513, 5134, 6123, 6520, 6012, 7439, 7518, 8145, 7831, 9264, 9653, 8955, 10761, 11596

Offset: 1

Author

R. K. Guy, Oct 08 2002

Keywords

• nonn
• easy

Comments

Row sums of A063987. - R. J. Mathar, Jan 08 2015

prime(n) divides a(n) for n > 2. This is implied by a variant of Wolstenholme's theorem (see Hardy & Wright reference). - Isaac Saffold, Jun 21 2018

Examples

			If n = 3, then p = 5 and a(3) = 1 + 4 = 5. If n = 4, then p = 7 and a(4) = 1 + 4 + 2 = 7. If n = 5, then p = 11 and a(5) = 1 + 4 + 9 + 5 + 3 = 22. - _Michael Somos_, Jul 01 2018
		

References

• G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 4th ed., Oxford Univ. Press, 1960, p. 88-90.
• Kenneth A. Ribet, Modular forms and Diophantine questions, Challenges for the 21st century (Singapore 2000), 162-182; World Sci. Publishing, River Edge NJ 2001; Math. Rev. 2002i:11030.

Links

• Zak Seidov, Table of n, a(n) for n = 1..1000
• Christian Aebi and Grant Cairns, Sums of Quadratic residues and nonresidues, arXiv preprint arXiv:1512.00896 [math.NT] (2015).

Crossrefs

Cf. A076410.

Sums of residues, nonresidues, and their differences, for p == 1 mod 4, p == 3 mod 4, and all p: A171555; A282035, A282036, A282037; A076409, A125615, A282038.

Programs

Maple

A076409 := proc(n)
  local a,p,i ;
  p := ithprime(n) ;
  a := 0 ;
  for i from 1 to p-1 do
    if numtheory[legendre](i,p) = 1 then
       a := a+i ;
    end if;
  end do;
  a ;
end proc: # R. J. Mathar, Feb 26 2011

Mathematica

Join[{1,1}, Table[ Apply[ Plus, Flatten[ Position[ Table[ JacobiSymbol[i, Prime[n]], {i, 1, Prime[n] - 1}], 1]]], {n, 3, 48}]]
Join[{1}, Table[p=Prime[n]; If[Mod[p,4]==1, p(p-1)/4, Sum[PowerMod[k,2, p],{k,p/2}]], {n,2,1000}]] (* Zak Seidov, Nov 02 2011 *)
a[ n_] := If[ n < 3, Boole[n > 0], With[{p = Prime[n]}, Sum[ Mod[k^2, p], {k, (p - 1)/2}]]]; (* Michael Somos, Jul 01 2018 *)

PARI

a(n,p=prime(n))=if(p<5,return(1)); if(k%4==1, return(p\4*p)); sum(k=1,p-1,k^2%p) \\ Charles R Greathouse IV, Feb 21 2017

Formula

If prime(n) = 4k+1 then a(n) = k*(4k+1).

For n>2 if prime(n) = 4k+3 then a(n) = (k - b)*(4k+3) where b = (h(-p) - 1) / 2; h(-p) = A002143. For instance. If n=5, p=11, k=2, b=(1-1)/2=0 and a(5) = 2*11 = 22. If n=20, p=71, k=17, b=(7-1)/2=3 and a(20) = 14*71 = 994. - Andrés Ventas, Mar 01 2021

Extensions

Edited and extended by Robert G. Wilson v, Oct 09 2002