A circular orbit closes only when speed-square over radius equals the field. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Candidate acceleration 1
Radius 4 and speed-square 4 give centripetal acceleration 1.
rv2=44=1,a=1
Candidate acceleration 4
Radius 2 and speed-square 8 give centripetal acceleration 4.
rv2=28=4,a=1
Candidate acceleration 2
Radius 2 and speed-square 4 give centripetal acceleration 2.
rv2=24=2,a=0
Circular closure requires v-square over radius to equal the field
The first two rows close exactly. The third row uses the same radius as row two but halves the speed-square, so its acceleration is too small.