What is bond convexity?how is it calculated?
convexity is a measure of the sensitivity of the duration of a bond to changes in interest rates.
Calculation of convexity
Convexity is a measure of the curvature or 2nd derivative of how the price of a bond varies with interest rate, i.e. how the duration of a bond changes as the interest rate changes. Specifically, one assumes that the interest rate is constant across the life of the bond and that changes in interest rates occur evenly. Using these assumptions, duration can be formulated as the first derivative of the price function of the bond with respect to the interest rate in question. Then the convexity would be the second derivative of the price function with respect to the interest rate.
Application of convexity
1. Convexity is a risk management figure, used similarly to the way 'gamma' is used in derivatives risks management; it is a number used to manage the market risk a bond portfolio is exposed to. If the combined convexity of a trading book is high, so is the risk. However, if the combined convexity and duration are low, the book is hedged, and little money will be lost even if fairly substantial interest movements occur. (Parallel in the yield curve.)
2. The second-order approximation of bond price movements due to rate changes uses the convexity:
\Delta(B) = B\left[\frac{C}{2}(\Delta®)^2 - D\Delta®\right].
So the higher the rating or credibility of the issuer the less the convexity and the less the gain from risk-return game or strategies; after all less convexity means less price-volatility or risk, less risk means less return.
More convex a portfolio higher the risk content.
Algebraic definition
If the flat floating interest rate is r and the bond price is B, then the convexity C is defined as
C = \frac{1}{B} \frac{d^2\left(B®\right)}{dr^2}.
Another way of expressing C is in terms of the duration D:
\frac{d}{dr} B ® = -DB.
Therefore
CB = \frac{d(-DB)}{dr} = (-D)(-DB) + \left(-\frac{dD}{dr}\right)(B),
leaving
C = D^2 - \frac{dD}{dr}.
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