Suppose that whether or not it rains today depends on previous weather

conditions through the last two days.

Specifically , suppose that

if it hasrained for the past two days,then it will rain tommorow with probability0.7;

if itrained today but not yesterday, then i will rain tommorow with probabilityo.5;

if itrained yesterday but not today, then it will rain tommorow with probability0.4;

if it hasnot rained in the past two days, then it will rain tommorow with probability0.2.

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the transition matrix

P = $\displaystyle \begin{bmatrix}0.7&0&0.3&0\\0.5&0&0.5&0\\0&0.4&0&0 .6\\0&0.2&0&0.8 \end{bmatrix}$

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i dont understand the mechanism to obtain the transition matrix in such a pattern............

why there are zeros? what are the meanings behind them?