The concept of choosing "more" sparse bases or CS sensing matrix lies in the measure of mutual coherence. It is defined as follows for a given matrix at order k.

$$ M(A, k) = max_{p} max_{p \ne q, q \in \Omega } \sum_{q} | <a_{p}, a_{q}> | / ( |a_{p}| |a_{q}|)$$ When k=1, it is easy to understand what it means. Basically we find the largest inner product among columns of the given matrix. Lower the value better the sparsity i.e. incoherence. However a single number may not be so informative, after all how low is better. With the definition of David Donoho and Joel Tropp, if M is slowly increasing then matrix said to be enchances sparsity. Larger value of k forms a set of columns $\Omega$, and the second colums are selected from this set i.e. second max argument in the above definition.

In a recent post I have shortly reviewed my R package for CS called R1magic. Its recent version 0.2 contains a functionality to compute $M(A, k)$. Also now there is a public Github repository of the package. mutualCoherence function is written fully functional way. All operations for computing $M(A,k)$ performed in vectorial fashion in R, using function closures and apply. However, for much larger matrices, a low level implementation may be required.

**Example**

Here we shortly investigate coherence of Fourier, random and mixed bases in R.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | require("R1magic") set.seed(42) A <- DFTMatrix0(10) # Fourier Bases B <- matrix(rnorm(100), 10, 10) # Gaussian Random Matrix C <- A %*% B # A sensing matrix with A and B as above aa<-mutualCoherence(A, 8) bb<-mutualCoherence(A, 8) bb<-mutualCoherence(A, 8) aa [1] 1 1 1 1 1 1 1 1 bb [1] 0.6784574 1.2011489 1.7001046 2.1713561 2.4664608 2.7302690 2.7908302 [8] 2.9623327 cc [1] 0.7506222 1.3448452 1.8047043 2.1105348 2.3350516 2.4703822 2.5898766 [8] 2.6882250 |

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