有限等距常数（RestrictedIsometry Constant, RIC）是与有限等距性质（Restricted IsometryProperty, RIP）紧密结合在一起的一个参数。
一、RIC定义：
在前面的一篇RIP文章中其实已经提到了，这里在贴出该定义：
CS满足条件：S阶RIP性质只要要求0<δS<1就可以了，而RIC是指满足RIP的最小δS。
二、RIC计算：
RIC与特征值的关系：
三、MATLAB代码
function y=RIPText(x,t)
% calculete the kth Restricted Isometry Constant of measurement matrix x.
% Modified on March 13th, .
% Reference: Wei Dai, Olgica Milenkovic. Subspace Pursuit for Compressive Sensing
% Signal Reconstruction. IEEE Trans. ona Information Theory. vol., no.,
% -. May,
%calculate the Restricted Isomentry Constant of matrix constructed by
%random n columns of the original matrix x.
%created by Li Bo in March 16th,
%Harbin Institute of Technolgy
n=size(x,);%the numbers of column of original matrix x
Num=:n;%create a row of numbers from one to n with iterval
Com=combntns(Num,t);% all the combination of the selected t columns from total n columns
ma=size(Com,);% the number of combinations
delta=zeros(,ma);% a vector used to store the restricted isometry constant candidate
for i=: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
用此代码来验证矩阵其实是不现实的，原因解释如下，在程序运行过程中容易导致内存不足等问题。