Finding Volume Traversed by Rt from a to b

[bomba923]

(Wow...it's been over three months since I posted anything...)
Anyhow,

Given continuous functions
\begin{gathered}<br /> f_1 :\left[ {a,b} \right] \to \mathbb{R}^3 \hfill \\<br /> \vdots \hfill \\<br /> f_n :\left[ {a,b} \right] \to \mathbb{R}^3 \hfill \\ <br /> \end{gathered}
for which
\exists g:\left[ {a,b} \right] \to \mathbb{R}^3 {\text{ such that }}f_1 \cdot g = \cdots = f_n \cdot g

define \forall t \in \left[ {a,b} \right]
R_t = \left\{ {\left( {x,y,z} \right)\left| {\left( {x,y,z} \right) \cdot g = f_1 \cdot g} \right.} \right\} \cap
\bigcup\limits_{\begin{subarray}{l} <br /> j &lt; k &lt; m \leqslant n, \\ <br /> \left( {j,k,m} \right) \in \mathbb{N}^3 <br /> \end{subarray}} {\left\{ {\left( {x,y,z} \right)\left| \begin{gathered}<br /> \left( {y - y_j } \right)\left( {x_j - x_k } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_k } &amp; {y_j - y_k } \\<br /> {x_j - x_m } &amp; {y_j - y_m } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_j } \right)\left( {y_j - y_k } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_k } &amp; {y_j - y_k } \\<br /> {x_j - x_m } &amp; {y_j - y_m } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {y - y_j } \right)\left( {x_j - x_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_m } &amp; {y_j - y_m } \\<br /> {x_j - x_k } &amp; {y_j - y_k } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_j } \right)\left( {y_j - y_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_m } &amp; {y_j - y_m } \\<br /> {x_j - x_k } &amp; {y_j - y_k } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {y - y_k } \right)\left( {x_k - x_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_k - x_m } &amp; {y_k - y_m } \\<br /> {x_k - x_j } &amp; {y_k - y_j } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_k } \right)\left( {y_k - y_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_k - x_m } &amp; {y_k - y_m } \\<br /> {x_k - x_j } &amp; {y_k - y_j } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_j } \right)\left( {x_j - x_k } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_j } \right)\left( {z_j - z_k } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_j } \right)\left( {x_j - x_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_j } \right)\left( {z_j - z_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_k } \right)\left( {x_k - x_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_k - x_m } &amp; {z_k - z_m } \\<br /> {x_k - x_j } &amp; {z_k - z_j } \\<br /> <br /> \end{array} } \right| \leqslant \left( {x - x_k } \right)\left( {z_k - z_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_k - x_m } &amp; {z_k - z_m } \\<br /> {x_k - x_j } &amp; {z_k - z_j } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_j } \right)\left( {y_j - y_k } \right)\left| {\begin{array}{*{20}c}<br /> {y_j - y_k } &amp; {z_j - z_k } \\<br /> {y_j - y_m } &amp; {z_j - z_m } \\<br /> <br /> \end{array} } \right| \leqslant \left( {y - y_j } \right)\left( {z_j - z_k } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_j } \right)\left( {y_j - y_m } \right)\left| {\begin{array}{*{20}c}<br /> {y_j - y_m } &amp; {z_j - z_m } \\<br /> {y_j - y_k } &amp; {z_j - z_k } \\<br /> <br /> \end{array} } \right| \leqslant \left( {y - y_j } \right)\left( {z_j - z_m } \right)\left| {\begin{array}{*{20}c}<br /> {x_j - x_m } &amp; {z_j - z_m } \\<br /> {x_j - x_k } &amp; {z_j - z_k } \\<br /> <br /> \end{array} } \right| \wedge \hfill \\<br /> \left( {z - z_k } \right)\left( {y_k - y_m } \right)\left| {\begin{array}{*{20}c}<br /> {y_k - y_m } &amp; {z_k - z_m } \\<br /> {y_k - y_j } &amp; {z_k - z_j } \\<br /> <br /> \end{array} } \right| \leqslant \left( {y - y_k } \right)\left( {z_k - z_m } \right)\left| {\begin{array}{*{20}c}<br /> {y_k - y_m } &amp; {z_k - z_m } \\<br /> {y_k - y_j } &amp; {z_k - z_j } \\<br /> <br /> \end{array} } \right| \hfill \\ <br /> \end{gathered} \right.} \right\}}
where \forall i &gt; 0,\;\left( {x_i ,y_i ,z_i } \right) = f_i \left( t \right)

Find the net volume traversed by R_t from t=a to t=b if
\exists p,q \in \left( {a,b} \right):\left( {R_p - \partial R_p } \right) \cap \left( {R_q - \partial R_q } \right) \ne \emptyset \, \wedge \, p \ne q

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*Edit: it may be simpler to describe R_t in words: R_t is the "union of all closed triangular regions defined by vertices f_j,f_k,f_m for all combinations of j,k,m at any t \in \left[ {a,b} \right]."

 

[bomba923]

Anybody?

Well, let's take the simplest case (n=3), where we have three continuous functions
\begin{gathered}<br /> f_1 :\left[ {a,b} \right] \to \mathbb{R}^3 \hfill \\<br /> f_2 :\left[ {a,b} \right] \to \mathbb{R}^3 \hfill \\<br /> f_3 :\left[ {a,b} \right] \to \mathbb{R}^3 \hfill \\ <br /> \end{gathered}
and
\forall t \in \left[ {a,b} \right], let R_t be the open triangular region with vertices f₁(t), f₂(t), and f₃(t).

To simplify matters, assume that \forall p,q \in \left( {a,b} \right),\;\left( {R_p \cap R_q \ne \emptyset } \right) \to \left( {p = q} \right).
Find the net volume traversed by R_t from t=a to t=b
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Initially, one might guess the net volume to simply the sum of the areas of R_t:
V_{net} = \frac{1}{2}\int\limits_a^b {\left\| {\left( {f_1 \left( t \right) - f_2 \left( t \right) } \right) \times \left( {f_1 \left( t \right) - f_3 \left( t \right) } \right)} \right\|dt}
But, that is false!
Consider
\begin{gathered}<br /> f_1 \left( t \right) = \left( {t,0,0} \right) \hfill \\<br /> f_2 \left( t \right) = \left( {0,t,0} \right) \hfill \\<br /> f_3 \left( t \right) = \left( {0,0,t} \right) \hfill \\ <br /> \end{gathered}
for
0 \leqslant t \leqslant 1

The region traversed by R_t is the tetrahedron with vertices (0,0,0), (1,0,0), (0,1,0), (0,0,1).
Its volume is simply
\int\limits_0^1 {\int\limits_0^{1 - x} {\left( {1 - x - y} \right)dy} dx} = \frac{1}{6}
However,
\frac{1}{2}\int\limits_0^1 {\left\| {\left( {f_1 \left( t \right) - f_2 \left( t \right)} \right) \times \left( {f_1 \left( t \right) - f_3 \left( t \right)} \right)} \right\|dt} = \frac{{\sqrt 3 }}{6} \ne \frac{1}{6}
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So, given three continuous functions
\begin{gathered}<br /> f_1 :\left[ {a,b} \right] \subset \mathbb{R} \to \mathbb{R}^3 \hfill \\<br /> f_2 :\left[ {a,b} \right] \subset \mathbb{R} \to \mathbb{R}^3 \hfill \\<br /> f_3 :\left[ {a,b} \right] \subset \mathbb{R} \to \mathbb{R}^3 \hfill \\ <br /> \end{gathered}

How can I calculate the net volume traversed by R_t?
(That is, is there a general formula by which one can calculate this volume?)