equilibrium index in an array (#9856)

* equilibrium index in an array

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* equilibrium index in an array

* equilibrium index in an array

* equilibrium index in an array removed type in docstring

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Co-authored-by: ArunSiva <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>