# fourth order moment condition
# for details, see He and Ter\"{a}svirta (2004) and Nakatani and Ter\"{a}svirta (2007).
fourth <- function(A,B,R){
AB <- A+B
AA <- A%x%A
G <- AB%x%AB + 2*AA*(diag(as.vector(R))^2)
max(Mod(eigen(G)$values))
}
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