function f=polfun_covb(x,xe,c,k,q,pfi)

% Written by John L. Crassidis 9/03

% Function Routine for Van der Pol's Covariance Equation in Backward Integration

% State Matrix
fpart=[0 1;-4*c*xe(1)*xe(2)-k -2*c*(xe(1)^2-1)];

% Covariance
p=[x(1) x(2);x(3) x(4)];
pdot=-[fpart+q*pfi]*p-p*[fpart+q*pfi]'+q;
f=[pdot(1,1);pdot(1,2);pdot(2,1);pdot(2,2)];