  The g factor of lithiumlike 28Si11+ has been measured in a triple-Penning
trap with a relative uncertainty of 1.1x10^{-9} to be g_exp=2.0008898899(21).
The theoretical prediction for this value was calculated to be
g_th=2.000889909(51) improving the accuracy to 2.5x10^{-8} due to the first
rigorous evaluation of the two-photon exchange correction. The measured value
is in excellent agreement with the state-of-the-art theoretical prediction and
yields the most stringent test of bound-state QED for the g factor of the
1s^22s state and the relativistic many-electron calculations in a magnetic
field.
