A 500 × 500 bus admittance matrix for an electric power system 4...
No. of non-zero elements = no. of diagonal element + 2 (no. of line)
4000 = 500 + 2 (lines)

A 500 × 500 bus admittance matrix for an electric power system 4...
A 500 is a term that is often used to refer to a 500 error or HTTP status code 500. It is an error message that is displayed when there is an internal server error. This error message is typically displayed when the server encounters an unexpected condition that prevents it from fulfilling the request made by the client.
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When encountering a 500 error, it is recommended to contact the website administrator or the server administrator to report the issue. They will usually investigate the error and take the necessary steps to resolve it. In some cases, the error may be temporary, and refreshing the page or trying again later may resolve the issue.