We find that $2$N current loops are coplanar and coaxial. The first has radius a and current I. The second has radius $2$a and current $2$I, and the pattern is repeated up to the Nth$,$ which has radius Na and current NI$.$ The current in the loops alternates in direction from loop to loop as seen from above. Thus the current in the first loop is counterclockwise, in the next clockwise, up to the last loop where it is again clockwise. The magnitude of the magnetic field at the center of the loops is

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