Satoshi Math mistake?

On page 7, where Satoshi shows the Poisson distribution, the probability that the attacker can reach and find a malicious node and reduce the gap by -1.

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He has the Poisson formula, multiplied by each attacker's progress, where there is a split function. If k <= z, Qz applies where (Z - K), but in that case, wouldn't K be less STRICT than Z ? since if K <= Z, and if K = Z, then (Z-K) = 0, giving rise to the division of a 1, and it would go to the party case below, creating a contradiction, since the one below gives 1 always for K > Z, but the other K = Z also gives 1, and K is not > Z in this case.

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Shouldn't it be something like, the first case IF K < Z , and the second case be K >= Z ?