(1)
where (u, 0) are the surface parameters of .
The family of surfaces, is generated in and is represented as (2)
where matrix , describes the coordinate transformation from , to .
We assume that the family of surfaces , obtained by vector equation (2) satisfies the
following requirements in the space
(1)
(2) Parameters are independent (they are not related by a function or and ,, is indeed
a two-parameter family of surfaces.
(3) Vector function where are the given values represents a regular surface, that is
represented as .
Taking into account the requirement mentioned above, we say that
(3)
And are independent.
3. Necessary condition of envelope existence applied in differential
geometry
Assume that a set of parameters
1
Σ
ϕ
φ
,
Σ
1
Σ
2
S
21
M
1
S
2
S
ψ
φ
,
Σ
:
)
,
(
,
)
,
(
A
E
u
∈
∈
ψ
φ
θ
)
,
(
ψ
φ
)
(
ψ
φ
))
(
φ
ψ
ψ
φ
,
Σ
)
,
,
,
(
1
1
2
ϕ
φ
θ
u
r
)
,
(
1
1
ψ
φ
)
,
(
1
1
ψ
φ