A000074 Number of odd integers <= 2^n of form x^2 + y^2.

1, 1, 2, 4, 7, 13, 25, 43, 83, 157, 296, 564, 1083, 2077, 4006, 7733, 14968, 29044, 56447, 109864, 214197, 418080, 816907, 1598040, 3129063, 6132106, 12027122, 23606527, 46366165, 91127332, 179207074, 352615528, 694182554, 1367278759

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn

Comments

First differences of A000050. - Jean-François Alcover, Mar 19 2014

Examples

			a(4)=4 since 2^4=16 and 1=1^2, 5=1^2+2^2, 9=3^2, 13=2^2+3^2.
		

References

• N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
• N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

• Seth A. Troisi, Table of n, a(n) for n = 1..50
• Daniel Shanks, The second-order term in the asymptotic expansion of B(x), Math. Comp., 18 (1964), 75-86.
• Index entries for sequences related to populations of quadratic forms

Crossrefs

Cf. A000050.

Programs

PARI

a(n)=if(n<0,0,sum(k=1,2^(n-1),0
				

Extensions

9 more terms from Sean A. Irvine, Sep 14 2009