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The current MatrixRank is a slight foolish without any capacity of symbolic computation ,feature of Mathematica, like this:

(mat = Normal@SparseArray[{i_, i_} -> a, 5, b]) // MatrixForm

enter image description here

MatrixRank[mat]
(*5*)

But actually we expect the result in mathematics is:

when a=b=0,rank(mat)=0
when a=b≠0,rank(mat)=1
when a+4b=0,rank(mat)=4
when a≠b&&a≠-4b,rank(mat)=5

I'd like to be Solve's result:

In[1]:= Solve[x^2 + y^2 + x == 1, y, Reals]

Out[1]= {{y -> 
   ConditionalExpression[-Sqrt[1 - x - x^2], 
    1/2 (-1 - Sqrt[5]) < x < 1/2 (-1 + Sqrt[5])]}, {y -> 
   ConditionalExpression[Sqrt[1 - x - x^2], 
    1/2 (-1 - Sqrt[5]) < x < 1/2 (-1 + Sqrt[5])]}}

It will give a ConditionalExpression for every result.And I think the Mathematica can do it via some solution.like following we can find some condition for this:

enter image description here

But I don't know what I should to do next for this.So help Mathematica,help me to improve or rewrite the MatrixRank,give some thinking,please.

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    $\begingroup$ First you will want to know what makes eigenvalues vanish. That can be found as follows. In[10]:= roots = x /. Solve[CharacteristicPolynomial[mat, x] == 0, x] Out[10]= {a - b, a - b, a - b, a - b, a + 4 b}. Next take all subsets and invoke something like Solve[subset==0&&Complement[set,subset]!=0] to find all cases where rank= #subset. In this case of course it simplifies since there is a repeated root, but this should give the general idea. $\endgroup$ – Daniel Lichtblau Oct 15 '15 at 14:30
  • $\begingroup$ Can you post an arranged answer that I can accept it?And it will be helpful to another to read it. $\endgroup$ – yode Oct 15 '15 at 14:51
  • $\begingroup$ One other possibility is similar to Daniel's solution, except that what I had in mind was to use LUDecomposition[], and then do the checks described by Daniel on the diagonal entries of the upper triangular factor. $\endgroup$ – J. M. will be back soon♦ Oct 15 '15 at 15:15
  • $\begingroup$ The approach by @J.M. might be more sensible since it reduces the possibility of working over an algebraic extension to the case of finally checking subsets. But I think there is then a new complication, of rechecking such cases to see what happens next (heuristic reason: if a potential pivot were to vanish, that does not mean there is no other possible pivot). $\endgroup$ – Daniel Lichtblau Oct 15 '15 at 15:27
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This is not a complete solution but should provide a start. In the special case of a square matrix we can determine conditions on rank by determining where specific numbers of eigenvalues vanish. We show how to set this up using the example from the post.

mat = Normal@SparseArray[{i_, i_} -> a, 5, b]

(* Out[17]= {{a, b, b, b, b}, {b, a, b, b, b}, {b, b, a, b, b}, {b, b, b,
   a, b}, {b, b, b, b, a}} *)

roots = x /. Solve[CharacteristicPolynomial[mat, x] == 0, x]

(* Out[18]= {a - b, a - b, a - b, a - b, a + 4 b} *)

To proceed from here one could do as follows.

For each subset sroots of roots, do Reduce[Flatten[Thread[sroots==0],Thread[Complement[roots,sroots]!=0]]]. If sroots has size n then this gives a set of conditions for the rank to be exactly n.

This will take some real work to make efficient in those cases where, as above, there are (generically) repeated roots.

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  • $\begingroup$ Does x /. Solve[CharacteristicPolynomial[mat, x] == 0, x] do anything that Eigenvalues[mat] can't do? $\endgroup$ – J. M. will be back soon♦ Oct 15 '15 at 16:03
  • $\begingroup$ @J.M. No, of course not, I'm just fuzzy this morning. $\endgroup$ – Daniel Lichtblau Oct 15 '15 at 16:33
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Not perfect but a good start

Join[{#, MatrixRank[mat /. #]} & /@ 
  Flatten[{ToRules[
     Reduce[Det[mat] == 0, a]]}], {If[(fullRank = 
      Reduce[Det[mat] > 0, a, Reals]) =!= False, {fullRank, 5}, 
   Nothing]}]

{{a -> -4 b, 4}, {a -> b, 1}, {(b <= 0 && a > -4 b) || (b > 0 && (-4 b < a < b || a > b)), 5}}

But I don't know how to deal the case when rank is $0$

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