\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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