1,1

2nX2n spirals of the form:

(Example of n = 2)

7...8...9...10

6...1...2...11

5...4...3...12

16..15..14..13

a(n) = 2n + 4*floor((n-1)^2/4) = 2n + 4*A002620(n-1)

a(1) = 2(1) + 4*floor((1-1)/4) = 2

a(2) = 2(2) + 4*floor((2-1)/4) = 4

(Python) a = lambda n: 2*n + 4*floor((n-1)**2/4)

Cf. A002061 (odd diagonal), A002620.

Sequence in context: A039682 A111149 A123689 this_sequence A006584 A032246 A077627

Adjacent sequences: A137925 A137926 A137927 this_sequence A137929 A137930 A137931

nonn,easy

William A. Tedeschi (fynmun(AT)hotmail.com), Feb 29 2008