According to the second edition (1989) of the Oxford English Dictionary it is not improper in British English to use the variant "infeasible". The OED does say that "infeasible" is "rare", but it provides examples of its having been used as recently as 1881. Its most recent example of the use of the variant "unfeasible" dates from 1886, which does not do much to demonstrate the comparative rarity of "infeasible", since there is only five years between them. To call "infeasible" unacceptable seems much too emphatic.
According to the second edition (1989) of the Oxford English Dictionary it is not improper in British English to use the variant "infeasible". The OED does say that "infeasible" is "rare", but it provides examples of its having been used as recently as 1881. Its most recent example of the use of the variant "unfeasible" dates from 1886, which does not do much to demonstrate the comparative rarity of "infeasible", since there is only five years between them. To call "infeasible" unacceptable seems much too emphatic.
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