Abstract
The first two terms in the Z-expansion of matrix elements of the electric dipole transition operator for all 1snl 1,3L-1sn'l' 1,3L' transitions in the helium isoelectronic sequence with n,n'<or=4 are calculated. It is demonstrated that an arbitrary screening parameter in a modification of the screening approximation of Dalgarno and Stewart (1960) can be chosen to increase the rate of convergence of the expansion. A practical procedure for choosing the arbitrary screening parameter is presented.

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