In-and-Out Conversions
Byan in-and-out conversion we mean that a floating-point
number in one base is converted into 
a floating-point number in another base and then converted
back to a floating-point number in the original 
base.  For all combinations of rounding and truncation
conversions the question is considered of how 
many significant digits are needed in the intermediate
base to allow such in-and-out conversions to return 
the original number (when possible), or at least significant digit.
CACM January, 1968
Matula, D. W.
