TBILLEQ is a function in Google Sheets that calculates the equivalent annualized yield of a US Treasury bill, given its settlement date, maturity date, and discount rate. The function returns a decimal number representing the annualized yield of the Treasury bill as a percentage.

The settlement date is the date on which the Treasury bill is purchased, while the maturity date is the date on which it matures. The discount rate is the rate at which the Treasury bill is sold at a discount to its face value.

The TBILLEQ function uses an iterative method to calculate the yield, which means that it will keep calculating until it finds a solution that is accurate within a certain tolerance. This function is useful for investors who want to compare the yields of different Treasury bills and make informed investment decisions.