有限等距常數（RestrictedIsometry Constant, RIC）是與有限等距性質（Restricted IsometryProperty, RIP）緊密結合在一起的一個參數。
一、RIC定義：
在前面的一篇RIP文章中其實已經提到了，這里在貼出該定義：
CS滿足條件：S階RIP性質只要要求0<δS<1就可以了，而RIC是指滿足RIP的最小δS。
二、RIC計算：
RIC與特征值的關系：
三、MATLAB代碼
function y=RIPText(x,t) % calculete the kth Restricted Isometry Constant of measurement matrix x. % Modified on March 13th, 2012. % Reference: Wei Dai, Olgica Milenkovic. Subspace Pursuit for Compressive Sensing % Signal Reconstruction. IEEE Trans. ona Information Theory. vol.55, no.5, % 2230-2249. May, 2009 %calculate the Restricted Isomentry Constant of matrix constructed by %random n columns of the original matrix x. %created by Li Bo in March 16th, 2011 %Harbin Institute of Technolgy n=size(x,2);%the numbers of column of original matrix x Num=1:n;%create a row of numbers from one to n with iterval 1 Com=combntns(Num,t);% all the combination of the selected t columns from total n columns ma=size(Com,1);% the number of combinations delta=zeros(1,ma);% a vector used to store the restricted isometry constant candidate for i=1:ma b=x(:,Com(i,:));% new matrix constructed by the t selected columns e=eig(b'*b-eye(t));%calculate all the eigen values of matrix b'*b-eye(t) delta(i)=(max(abs(e)));%select the largest absolute eigen value as restricted isomentry candidate end y=max(delta);% select the largest one as restricted isometry constant
用此代碼來驗證矩陣其實是不現實的，原因解釋如下，在程序運行過程中容易導致內存不足等問題。
