For the following code,
X=[2 2 3 100 4 0 98 100 90 2 3 67 98 0 6 6 89 9 21 78] [C,L]=wavedec(X,N,'db1');
N is the level , I get n(approximation and detail coefficients)>n(X)
What does this signify?
Why does this increase happen?
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This may be caused by "border effects" related to subsampling and signal length with perfect reconstruction filter banks.
For 2-band wavelets, there are standard non-expansive solutions (for instance with periodic border extension) when $2^N$ divides the length $L$ of your signal. In your case, one or two levels work without length increase, becauses $20=20/2+20/2 = (10/2+10/2)+10$. Then, starting at level $3$, you cannot divide $5$ into equal parts, since $2^3 = 8$ does not divide 20.
For some applications (outside compression) this is not really an issue.