Arduino Matrix Multiplication with Eigen

Question

I am using Eigen library with Arduino Mega 2560. I am creating dynamic matrices using Eigen library but it either returns nothing or the matrix itself.

#include <stlport.h>
#include <Eigen30.h>
#include <Eigen/LU>
#include <Eigen/Dense>

Eigen::MatrixXd b, c;

void setup() {
  Serial.begin(9600);
  Eigen::MatrixXd a;

  for(int i=0; i < 4; i++){
    for(int j=0; j < 4;j++){
      a(i,j) = 1;
    }
  }

  c = a * a;

}

void loop() {

}

This code returns

1 1 1 1 
1 1 1 1
1 1 1 1 
1 1 1 1 

If the matrix was defined as Eigen::MatrixXd a(4,4) Then the code will not return anything. What is wrong here?

Thanks.