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Thursday, October 18, 2012

On an integral geometric formula

\newcommand{\bR}{\mathbb{R}} \newcommand{\bsV}{{\boldsymbol{V}}} \DeclareMathOperator{\Graffr}{\mathbf{Graff}^c}  \newcommand{\be}{\boldsymbol{e}} \newcommand{\bv}{\boldsymbol{v}}   \DeclareMathOperator{\Grr}{\mathbf{Gr}^c} \newcommand{\Gr}{\mathbf{Gr}} \newcommand{\Graff}{\mathbf{Graff}}

Suppose that \bsV is a finite dimensional real Euclidean  space, M\subset \bsV  is a smooth compact submanifold of dimension m and codimension r and we set


N:=\dim \bsV=m+r.

  For any  nonnegative integer c\leq \dim \bsV we denote by \Graff^c(\bsV) the  Grassmannian  of  affine subspaces of \bsV of codimension c,   by \Gr^c(\bsV) the Grassmannian of  codimension c vector subspaces of \bsV. We set \Gr_k(\bsV):=\Gr^{N-k}(\bsV).

The   codimension c Radon transform of a smooth function f: M\to  \bR  is a function

\widehat{f}:\Graff^c(\bsV)\to\bR ,

such that \newcommand{\eH}{\mathfrak{H}}

\widehat{f}(S) =\int_{S\cap M} f(x) d\eH^{m-c}(x),  \;\; \forall S\in \Graff^c(M), \label{r}\tag{R}

where d\eH^{m-c} denotes the (m-c)-dimensional Hausdorff measure.   If c\leq \dim M then a generic   affine plane S\in\Graff^c(\bsV) intersects  M transversally in which case the Hausdorff measure in (\ref{r}) is the usual Lebesgue measure induced my the  natural Riemann metric on S\cap M.

I want  to explain how to  recover the integral of f over M from its Radon transform.



Observe that we have an incidence set \newcommand{\eI}{\mathscr{I}}

\eI^c(\bsV) :=\Bigl\{ (\bv, S)\in \bsV\times \Graffr(\bsV);\;\; \bv\in S\;\Bigr\}

equipped with  natural projections

\bsV\stackrel{\lambda}{\leftarrow}\eI^c(\bsV)\stackrel{\rho}{\to}\Graffr(\bsV).\label{F}\tag{F}


For any subset X\subset \bsV we define

\eI^c(X):=\lambda^{-1}(M)\subset \eI^r(X),\;\; \Graffr(X)=\rho\Bigl(\;\eI^r(X)\;\Bigr).

Note that

\Graffr(X)=\Bigl\{ S\in \Graffr(\bsV);\;\; S\cap X\neq \emptyset\;\Bigr\}

and for any \bv\in\bsV we have

\lambda^{-1}(\bv) =\bigl\{ \bv+S;\;\;S\in \Grr(\bsV)\;\bigr\}=\Graffr(\bv)\subset \Graffr(\bsV).


Observe that  \eI^c(V)\to \bsV is a smooth fiber bundle  with fiber  \Gr^r(\bsV). In particular,  \eI^c(M)\to M is the bundle obtained  by restricting to the submanifold M.  Its fiber is also \Gr^c(\bsV).

At this point I need to recall some  basic facts described in great detail in Sections 9.1.2, 9.1.3 of  Lectures on the Geometry of Manifolds.

 The Grassmannain \Gr^c(\bsV) is equipped with a canonical O(\bsV)-invariant metric  with volume  density |d\gamma^c_\bsV| with total volume \newcommand{\sbinom}[2]{\genfrac{[}{]}{0pt}{}{#1}{#2}}

\int_{\Gr^c(\bsV)} |d\gamma_\bsV^c(L)|=\sbinom{N}{c},

where \sbinom{N}{c} is defined  in   equation (9.1.66) of the Lectures.

Now observe that  we have a natural projection \pi: \Graff^c(\bsV)\to \Gr^c(\bsV) that associates  to each affine  plane its translate through the origin.    A plane S\in\Graff^c(\bsV) intersects the orthogonal complement  of \pi(S) in a unique point C(S)=S\cap \pi(S)^\perp.   We obtain a an embeding

\Gr^c(\bsV)\ni S\mapsto \bigl(\;C(S), \pi(S)\;\bigr)\in \bsV\times\Gr^c(\bsV),\;\;C(S)\perp \pi(S),
\newcommand{\eQ}{\mathfrak{Q}}
and we  will regard  \Graff^c(\bsV) as a submanifold of \bsV\times \Gr^c(\bsV).  As such,     it becomes  the total space of  a vector bundle \eQ_c\to\Gr^c(\bsV), in fact a subbundle of the trivial bundle \bsV\times  \Gr^c(\bsV)\to\Gr^c(\bsV).   The orthogonal complement  \eQ_c^\perp of this bundle is the tautological vector bundle \newcommand{\eU}{\mathscr{U}}  {\eU}^c\to\Gr^c(\bsV).  In particular

\dim\Gr^c(\bsV)= c(N-c)+  c.

Along \Graff^c(\bsV) we have a canonical vector bundle,  the  vertical bundle VT\Graff^c(\bsV)\subset T\Graff^c(\bsV)   consisting of the kernels of d\pi, i.e., vectors tangent to the fibers of \pi. The  vertical bundle is equipped with a natural density  |d\bv|_c which when restricted to a fiber of \pi^{-1}(L)  induces the natural volume form on the fiber L^\perp viewed as a vector subspace of \bsV. As in Section 9.1.3 of the Lectures we define a product  density |d\tilde{\gamma}^c|=|d\tilde{\gamma}_\bsV^c| on \Graff^c(\bsV),

|d\tilde{\gamma}_\bsV^c|= |d\bv|_c\times \pi^*|d\gamma_\bsV^{c}|

Alternatively, the vector bundle \eQ_c, as a subbundle of the trivial bundle \bsV\times \Gr^c(\bsV)\to\Gr^c(\bsV)  is equipped with a natural metric connection. The  horizontal subbundle   HT\eQ_c\subset T\eQ_c  is isomorphic to \pi^* T\Gr^c(\bsV) and thus comes equipped with a natural metric.    The  vertical subbundle VT\eQ_c=VT\Graff^c(\bsV) is also equipped with a  natural  metric and in this fashion we obtain a metric on \Graff^c(\bsV)=\eQ_c. The density |d\tilde{\gamma}^c_\bsV|  is the volume density defined by this metric.


Suppose now that c\leq m=\dim M.   We denote by \Graff^c_*(M) the subset of \Graff^c(M) consisting of affine planes that intersect M transversally.    This is an open subset of \Graff^c(M).  The condition c\leq m implies that this set is nonempty.  (For c=1 this follows from the fact that the restriction to M of a generic linear function is a Morse function. Then look at iterated slicing by hyperplanes.)

Set

\eI^c_*(M)= \rho^{-1}\bigl(\;\Graff^c_*(M)\;\bigr)\subset \eI_M

The fiber of  \rho:\eI_*^c(M)\to \Graff^c_*(M) over S\in \Graff^c_*(M) is the submanifold S\cap M which is equipped with a metric density.  We obtain a density on \eI^c_*(M)

|d\nu^c_M|= |dV_{S\cap M}|\times \rho^*|d\tilde{\gamma}^c|. \tag{$\nu^c$}\label{nu}

If f: M\to\bR is a smooth function, then

\int_{\eI^c_*(M)}\lambda^*(f) |d\nu^c_M|=\int_{\Graff^c_*(M)}\left(\int_{S\cap M} f|dV_{S\cap M}\right) |d\tilde{\gamma}^c(S)|. \label{1}\tag{1}

For any  vector subspace U\subset \bsV) we denote by \Gr^c(\bsV)_U the set consisiting of subspaces L\in\Gr^c(\bsV) that intersect U transversely.


We now want to integrate \lambda^*(f) along the fibers of \lambda :\eI^c_*(M)\to M.  For any  vector subspace U\subset \bsV) we denote by \Gr^c(\bsV)_U the set consisting of subspaces L\in\Gr^c(\bsV) that intersect U transversely.

The fiber of this map over a point x\in M is an open  subset of x+\Gr^c(\bsV)_{T_xM}\subset \Graff^c(\bsV) with negligible complement.    The density |d\nu^c_M| on \eI^c_*(M) induces  a density

|d\nu^c_x|=|d\nu^M|/\lambda^*|dV_M|


on each fiber \lambda^{-1}(x) and we deduce

\int_{\eI^c_*(M)} \lambda^* f|d\nu^c(M)|= \int_M\left(\int_{\lambda^{-1}(x)}|d\nu^c_x|\right) f(x)|dV_N(x)|. \label{2}\tag{2}

The density |d\nu^c(x)| is  the restriction of a density |d\bar{\nu}^c_x| on \Gr^c(\bsV)_{T_xM}. In fact, a reasoning similar to the one   in the proof  of Lemma 9.3.21 in the Lectures implies that  for any U\in\Gr_m(\bsV) there exists a canonical density |d\bar{\nu}^c_U| on \Gr^c(\bsV)_U such that

T_*|d\nu^c_U|=|d\bar{\nu^c}_{T(U)}|,\;\;\forall T\in O(\bsV),\;\;U\in \Gr_m(\bsV), \label{3}\tag{3}


|d\bar{\nu}^c_x|=|d\bar{\nu}^c_{T_xM}|,\;\;\forall x\in M. \label{4}\tag{4}


 Using (\ref{3}) (\ref{4}) in (\ref{2}) we deduce that there  exists a constant Z=Z(N,m,c) that depends only on N,m,c such that

Z(N,m,c)=\int_{\nu^{-1}(x)} |d\bar{\nu}^c_x|,\;\;forall x\in M.

Using this in (\ref{2})  we conclude from (\ref{1}) that

Z(N,m,c)\int_{M}f(x)\; |dV_M(x)| =\int_{\Graff^c(\bsV)}\left(\int_{S\cap M} f(x)|dV_{S\cap M}|\right) |d\tilde{\gamma}^c_\bsV(S)|.\label{5}\tag{5}




To find the constant Z(N,m,c) we choose M and f judiciously.  We let M=\Sigma^m, the unit m-dimensional sphere contained in some (m+1)-dimensional subspace of \bsV.  Then, we let f\equiv 1. We deduce from (\ref{5}) that

Z(N,m,c)=\frac{1}{{\rm vol}\;(\Sigma^m)} \int_{\Graff^c(\bsV)} {\rm vol}\,(S\cap \Sigma^m)\;|d\tilde{\gamma}^c_\bsV(S)|.\label{6}\tag{6}


Using the Crofton formula in Theorem 9.3.34 in the Lectures  in the special case p=m-c we deduce


Z(N,m,c)=\sbinom{m}{c}.

Remark. 1        Consider the Radon transform


C_0^\infty(\bsV)\ni f\mapsto  \widehat{f}\in C^\infty\bigl(\;\Graff^c(\bsV)\;\bigr), \;\; \widehat{f}(S)=\int_S f(x)|dV_S(x)|,\;\;\forall S\in \Graff^c(\bsV).

Observe that \widehat{f} has compact support.   Indeed, if  the support of f is contained in a ball of radius R, then for any affine plane S\in \Graff^c(\bsV) such that {\rm dist}\,(0,S)>R we have \widehat{f}(S)=0.

Consider the dual Radon transform  \newcommand{\vfi}{\varphi}


C^\infty\bigl(\;\Graff^c(\bsV)\;\bigr)\ni \vfi\mapsto \check{\vfi}\in C^\infty(\bsV),\;\;\check{\vfi}(x)=\int_{\Gr^c(\bsV)} \vfi(x+L)\;|d\gamma^c(L)|,\;\;\forall x\in \bsV.



Consider the fundamental double fibration  (\ref{F}). Given f\in C_0^\infty(\bsV), \vfi\in C^\infty_0\bigl(\;\Graff^c(\bsV)\;\bigr) we obtain a function 


\Phi=\lambda^*(f)\cdot \rho^*(\vfi)\in C_0^\infty(\bsV)

Arguing as above, with M=\bsV  we  observe that \Graff^c_*(\bsV)=\Graff^c(\bsV) and we obtain as in (\ref{nu}) a density  |\nu^c_\bsV| on \eI^c_*(\bsV)=\eI^c(\bsV).   Denote by \rho_*\Phi |d\nu^c_\bsV| the   pushfoward  of the density \Phi|d\nu^c_\bsV. It is a density on \Graff^c(\bsV)  and we have the  Fubini formula (coarea formula)

\int_{\eI^c(\bsV)} \Phi(x,S) |d\nu^c_\bsV(x,S)|=\int_{\Graff^c(\bsV)}\rho_*\Phi|d\ni^c_\bsV|\label{7}\tag{7}

Similarly, we obtain

\int_{\eI^c(\bsV)} \Phi(x,S) |d\nu^c_\bsV(x,S)|=\int_{\bsV} \lambda_*\Phi |d\nu^c_\bsV|(x).\label{8}\tag{8}

From the construction  of |d\nu^c_\bsV| we deduce immediately that

\rho_*\Phi|d\nu^c_\bsV|(S)=  \widehat{f}(S) \vfi(S) |d\tilde{\gamma}^c|(S).


From the definitions of |d\nu^c_\bsV|, |d\gamma^c_\bsV| and  |d\tilde{\gamma}^c_\bsV| it follows easily that

\lambda_*\Phi |d\nu^c_\bsV|(x) =  f(x)\check{\vfi}(x)|dx|

Using the last equalities in (\ref{7}) and (\ref{8})  we deduce

\int_{\bsV} f(x)\check{\vfi}(x)=\int_{\Graff^c(\bsV)} \widehat{f}(S)\vfi(S) |d\tilde{\Gamma}^c(S)|. \tag{D}  \label{d}

The equality (\ref{d}) shows that the operations f\mapsto \widehat{f} and \vfi\mapsto \widehat{\vfi} are indeed dual to each other.   Note also that if we set  \vfi\equiv 1 in (\ref{d}) then

\check{\vfi}(x)={\rm vol}\,\bigl(\;\Gr^c(\bsV)\;\bigr)=\sbinom{N}{c}

and in this case we reobtain (\ref{5}) in the special case M=\bsV.  The  equality (\ref{d}) is  important for another reason.

Denote by C_0^{-\infty}(\bsV) the space of generalized functions with compact supports, then we can extend the Radon transform to such objects. If u\in C_0^{-\infty}(\bsV) then we define its   Radon transform \widehat{u} to be the compactly supported  generalized density on \Graff^c(\bsV) defined by the equality

\langle \widehat{u},\vfi\rangle=\langle u,\check{\vfi}\rangle ,\;\;\forall \vfi\in C^\infty\bigl(\;\Graff^c(\bsV)\;\bigr).

If M is a compact submanifold of \bsV, then we get a  Dirac-type  generalized function \delta_M on \bsV   defined by integration along M with respect to the volume density on M determined by the induced metric.   Then

\langle\widehat{\delta}_M,\vfi\rangle =\int_M  \check{\vfi}(x) |dV_M(x)|,\;\;\forall  C^\infty\bigl(\;\Graff^c(\bsV)\;\bigr).

The  generalized function \widehat{\delta}_M is represented by a locally integrable function

\widehat{\delta}_M(S) =\eH^{m-c}(M\cap S),\;\;\forall S\in \Graff^c(M).










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