Computational indistinguishability

In computational complexity, if ${\displaystyle \scriptstyle \{D_{n}\}_{n\in \mathbb {N} }}$ and ${\displaystyle \scriptstyle \{E_{n}\}_{n\in \mathbb {N} }}$ are two distribution ensembles indexed by a security parameter n (which usually refers to the length of the input), then we say they are computationally indistinguishable if for any non-uniform probabilistic polynomial time algorithm A, the following quantity is a negligible function in n:

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