%I A137928
%S A137928 2,4,10,16,26,36,50,64,82,100,122,144,170,196,226,256,290,324,362,400,
               442,
%T A137928 484,530,576,626,676,730,784,842,900,962,1024,1090,1156,1226,1296,1370,
               1444,
%U A137928 1522,1600,1682,1764,1850,1936,2026,2116,2210,2304,2402,2500,2602,2704,
               2810
%N A137928 The even principal diagonal of an 2nX2n spiral.
%C A137928 2nX2n spirals of the form:
%C A137928 (Example of n = 2)
%C A137928 7...8...9...10
%C A137928 6...1...2...11
%C A137928 5...4...3...12
%C A137928 16..15..14..13
%F A137928 a(n) = 2n + 4*floor((n-1)^2/4) = 2n + 4*A002620(n-1)
%e A137928 a(1) = 2(1) + 4*floor((1-1)/4) = 2
%e A137928 a(2) = 2(2) + 4*floor((2-1)/4) = 4
%o A137928 (Python) a = lambda n: 2*n + 4*floor((n-1)**2/4)
%Y A137928 Cf. A002061 (odd diagonal), A002620.
%Y A137928 Sequence in context: A039682 A111149 A123689 this_sequence A144834 A006584 
               A032246
%Y A137928 Adjacent sequences: A137925 A137926 A137927 this_sequence A137929 A137930 
               A137931
%K A137928 nonn,easy
%O A137928 1,1
%A A137928 William A. Tedeschi (fynmun(AT)hotmail.com), Feb 29 2008