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diff --git a/isCavityStable.m b/isCavityStable.m
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+function ret=isCavityStable (abcd_cavity) 
+    % finds if a cavity is stable baced on its abcd matrix
+    % see eq.4 for stability condition
+    % doi:10.1088/1464-4258/7/1/002
+    % http://dx.doi.org/10.1088/1464-4258/7/1/002
+
+    A=abcd_cavity(1,1);
+    B=abcd_cavity(1,2);
+    C=abcd_cavity(2,1);
+    D=abcd_cavity(2,2);
+
+    u= (A+D+2)/4
+    if ( (0 < u) && (u < 1) )
+        ret = true;
+    else
+        ret = false;
+    end
+
+end