What is cov(Yij,Ymn){“version”:”1.1″,”math”:”\(cov(Y_{ij},Y_…

What is cov(Yij,Ymn){“version”:”1.1″,”math”:”\(cov(Y_{ij},Y_{mn})\)”} where i≠m{“version”:”1.1″,”math”:”\(i \neq m\)”} and j≠n{“version”:”1.1″,”math”:”\(j \neq n\)”}?  This is the covariance between observations of two subjects from different classes.