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23. Polynomials
The monadic function`M=: 3: * ] ^ 2:` is a multiple of an integral power of its argument, and is called a monomial; and a sum of monomials such as`SM=: (3:*]^2:)+(2.5"_*]^4:)+(_5:*]^0:)` is a polynomial.
Any polynomial can be expressed in the standard form`c&p`,`` where`c` is a suitable list of coefficients, and where`p=: +/@([*]^i.@#@[)"1 0` . For example:

       SM=: (3:*]^2:)+(2.5"_*]^4:)+(_5:*]^0:)
       p=: +/@([*]^i.@#@[)"1 0
       c=: _5 0 3 0 2.5
       x=: _2 _1 0 1 2
       (SM x),(c p x),:(c&p x)
    47 0.5 _5 0.5 47
    47 0.5 _5 0.5 47
    47 0.5 _5 0.5 47

The primitive`p.` is equivalent to the function`p` defined above, and will be used hereafter. The polynomial`c&p.` is very important for a number of reasons, including:
1. It applies to any numeric argument, real or complex (and the parameter`c` may also be complex).
2. It can be used to approximate a wide range of functions.
3. It is closed under a number of operations; that is, the sum, difference, product, the composition`@`,`` the derivative, and the integral of polynomials are themselves polynomials.
4. The coefficients of the results of each case listed in 3 are easily expressed. For example, if`#c` equals`#d`,`` then`c&p. + d&p.` is equal to`(c+d)&p.` . More generally, it is equal to`(+/c,:d)&p.` . Thus:

    ps=: +/@,:                    Polynomial sum
    pd=: -/@,:                    Polynomial difference
    pp=: +//.@(*/)                Polynomial product
    D=: d.1                       Scalar (rank 0) first derivative
    pD=: 1: }. ] * i.@#           Polynomial derivative
    pI=: 0: , ] % 1: + i.@#       Polynomial integral

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