OFFSET
0,5
COMMENTS
The denominators in the expansion of (1-sqrt(1-x^2))/(1-x) are 1,1,2,2,8,8,16,16,... or 2^A005187(n) doubled. The sequence 0,1/2,1/2,5/8,5/8,... is the image of n under the Chebyshev related (rational) Riordan array c((x/2)^2),(x/2)c((x/2)^2)) with c(x) the g.f. of A000108. The image of n+1 under this array is 1,1,1,....
LINKS
G. C. Chingas, A. N. Garroway, W. B. Moniz, and R. D. Bertrand, Adiabatic J cross-polarization in liquids for signal enhancement in NMR, Journal of Chemical Physics, 102:8 (1980), 2526-2528 (page 1, equation (2)).
FORMULA
a(2n) = a(2n+1) = A120778(n-1) (conjectural). - Greg Martin, Aug 16 2014, corrected by M. F. Hasler, Aug 18 2014
PROG
(PARI) a(n) = my(y=x+O(x^(n+1))); numerator(polcoeff((1-sqrt(1-y^2))/(1-y), n)); \\ Michel Marcus, Aug 17 2014
CROSSREFS
KEYWORD
easy,frac,nonn
AUTHOR
Paul Barry, Jun 17 2008
STATUS
approved