I am asking about the stability of numerical methods. Specially pseudo spectral method.

when are solving fractional differential equations or for instance ordinary differential equations using Legendre operational matrices of integration and derivative. Then as clear that highest order derivative is assumed in product of two vectors coefficients vector and function vector, then operational matrices of derivative and integrals are used to obtain the lower order derivatives, and is then substituted in original equation, as a result we get system of algebraic equations,  

These algebraic equations are then solved to get approximation of solution differential equations. Clearly the use of large number of orthogonal polynomials will result in more accurate solution (as I experimentally observed over a large variety of problem).  

Now if some one is  asked that show that the algorithm is stable, and convergent. Then what does the term stable means here. In other words how can we theoretically prove that the algorithm is stable and convergent.  

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