Enter a complex number:

Complex is the Head for complex numbers:

Enter a purely imaginary number:

Even though there is no real part it has Head Complex:

The FullForm of a complex number x+Iy is Complex[x,y]:

Enter a complex number using the FullForm:

If the imaginary part is exactly zero, then the result is not Complex:

You have to use Re and Im to extract parts of Complex numbers:

Part does not work:

If either part of a complex number has machine precision, the entire number has machine precision:

Verify that the result is indeed a machine number:

Enter a complex number with an exact real part and an arbitrary-precision imaginary part:

The exactness is kept in computations when possible:

This is not, in general, possible:

Enter a complex number with arbitrary-precision real and imaginary parts:

The precision is based on the error in the complex plane:

_Complex can be used to stand for a complex number in a pattern:

A rule that switches real and imaginary parts:

An alternate definition:

Define a function over the complexes by using functions defined over the reals:

Multiply all pure imaginary numbers in an expression by a constant:

Note that the naive replacement Ia I would only multiply occurrences of I===Complex[0,1]:

Complexes are numbers:

Complexes are atomic objects with no subexpressions:

Use Complexes to indicate assumptions on domain conditions:

Real and imaginary parts of complex numbers can have different precisions:

Arithmetic operations will typically mix them:

But note that real and imaginary parts still have different precisions:

The precision of the whole number lies in between these two precisions:

Machine-precision evaluation of pure imaginary numbers yields an approximate zero real part:

Arbitrary‐precision evaluation yields an exact zero real part:

Numbers entered in the form x+Iy only become Complex numbers on evaluation:

The unevaluated form is expressed in terms of Plus and Times:

Evaluated complex numbers are atomic objects and do not explicitly contain I:

Patterns of the form Complex[x_,y_] can be used to match the whole complex number: