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Great explanation, @vincentt‌!
Another way to answer this question is to assign a coupon and face value to the security and compute all 3 yield metrics.
Let’s say the security costs 100 and pays 110 (100 face and 10 coupon) in 6 months.
The holding period yield would be:
HPY = (P1 – P0 + D1) / P0 = (100 – 100 + 10) / 100 = 10%
The bond-equivalent yield though is simply twice the semi-annual holding period yield, or 20%.
The effective annual interest rate for this security would be the annualized holding period yield:
r = (1 + 0.1)^2 – 1 = 21%