1.NounA grammatical number denoting a quantity of exactly two.
2.NounAn element of an algebra (the algebra of dual numbers) which includes the real numbers and an element ε which satisfies ε ≠ 0 and ε² = 0.
"If f(x) is a polynomial or a power series then its derivative can be obtained “automatically” using dual numbers as follows:
f
′
(
x
)
=
f
(
x
+
ϵ
)
−
f
(
x
)
ϵ
{\displaystyle f'(x)={f(x+\epsilon )-f(x) \over \epsilon }}
where
f
(
x
+
ϵ
)
{\displaystyle f(x+\epsilon )}
should be expanded using the algebra of dual numbers. The role of the ε is therein similar to that of an infinitesimal."