1. A method for determining, by means of a circuit, a result s k&plus;2 of an operation of a type 23 s k + 2 = ( s k &it; + &SmallCircle; &it; a k ) &it; + &SmallCircle; &it; a k + 1 k, a k, and a k&plus;1 are fractional signed operands and symbol 24 + &SmallCircle; a step of calculation of three sums (s, s′, s″) representative of a possible value of the result (s k&plus;2 ); and a step of selection of one of said three sums according to overflows having occurred in the sum calculation, wherein at least one step of the method uses a positive part and a negative part of at least one of the operands, the positive part of an operand being defined as being equal to the operand if the operand is a positive number and to zero otherwise, the negative part of an operand being defined as being equal to the operand if the operand is a negative number and to zero otherwise.