An almost unit is a nonunit in the integral domain of formal power series with a nonzero first coefficient, , where . Under the operation of composition, the almost units in the integral domain of formal power series over a field form a group (Henrici 1988, p. 45).
, where 
. Under the operation of composition, the almost units in the integral domain of formal power series over a field 
form a group (Henrici 1988, p. 45).