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Principal square root of −1
i in the complex or Cartesian plane. Real numbers lie on the horizontal axis, and imaginary numbers lie on the vertical axis.
The imaginary number i is defined solely by the property that its square is -1:
With i defined this way, it follows directly from algebra that i and -i are both square roots of -1.
Although the construction is called "imaginary", and although the concept of an imaginary number may be intuitively more difficult to grasp than that of a real number, the construction is perfectly valid from a mathematical standpoint. Real number operations can be extended to imaginary and complex numbers, by treating i as an unknown quantity while manipulating an expression (and using the definition to replace any occurrence of i2 with -1). Higher integral powers of i can also be replaced with -i, 1, i, or -1:
Being a quadratic polynomial with no multiple root, the defining equation x2 = -1 has two distinct solutions, which are equally valid and which happen to be additive and multiplicative inverses of each other. Once a solution i of the equation has been fixed, the value -i, which is distinct from i, is also a solution. Since the equation is the only definition of i, it appears that the definition is ambiguous (more precisely, not well-defined). However, no ambiguity will result as long as one or other of the solutions is chosen and labelled as "i", with the other one then being labelled as -i. After all, although -i and +i are not quantitatively equivalent (they are negatives of each other), there is no algebraic difference between +i and -i, as both imaginary numbers have equal claim to being the number whose square is -1.
In fact, if all mathematical textbooks and published literature referring to imaginary or complex numbers were to be rewritten with -i replacing every occurrence of +i (and therefore every occurrence of -i replaced by -(-i) = +i), all facts and theorems would remain valid. The distinction between the two roots x of x2 + 1 = 0, with one of them labelled with a minus sign, is purely a notational relic; neither root can be said to be more primary or fundamental than the other, and neither of them is "positive" or "negative".
In this case, the ambiguity results from the geometric choice of which "direction" around the unit circle is "positive" rotation. A more precise explanation is to say that the automorphism group of the special orthogonal groupSO(2, R) has exactly two elements: The identity and the automorphism which exchanges "CW" (clockwise) and "CCW" (counter-clockwise) rotations. For more, see orthogonal group.
All these ambiguities can be solved by adopting a more rigorous definition of complex number, and by explicitly choosing one of the solutions to the equation to be the imaginary unit. For example, the ordered pair (0, 1), in the usual construction of the complex numbers with two-dimensional vectors.
Consider the matrix equation Here, z2 + xy = -1, so the product xy is negative because xy = -(1 + z2), thus the point (x, y) lies in quadrant II or IV. Furthermore,
so (x, y) is bounded by the hyperbola xy = -1.
The imaginary unit is sometimes written in advanced mathematics contexts (as well as in less advanced popular texts). However, great care needs to be taken when manipulating formulas involving radicals. The radical sign notation is reserved either for the principal square root function, which is only defined for real x >= 0, or for the principal branch of the complex square root function. Attempting to apply the calculation rules of the principal (real) square root function to manipulate the principal branch of the complex square root function can produce false results:
The calculation rules
are only valid for real, positive values of a and b.
These problems can be avoided by writing and manipulating expressions like i, rather than . For a more thorough discussion, see square root and branch point.
The two square roots of i in the complex plane
The three cube roots of i in the complex plane
Just like all nonzero complex numbers, i has two square roots: they are[b]
Many mathematical operations that can be carried out with real numbers can also be carried out with i, such as exponentiation, roots, logarithms, and trigonometric functions. All of the following functions are complexmulti-valued functions, and it should be clearly stated which branch of the Riemann surface the function is defined on in practice. Listed below are results for the most commonly chosen branch.
^Some texts[which?] use the Greek letter iota (?) for the imaginary unit to avoid confusion, especially with indices and subscripts.
In electrical engineering and related fields, the imaginary unit is normally denoted by j to avoid confusion with electric current as a function of time, which is conventionally represented by i(t) or just i .
^To find such a number, one can solve the equation
(x + iy)2 = i
where x and y are real parameters to be determined, or equivalently
x2 + 2ixy - y2 = i.
Because the real and imaginary parts are always separate, we regroup the terms:
x2 - y2 + 2ixy = 0 + i
and by equating coefficients, real part and real coefficient of imaginary part separately, we get a system of two equations:
x2 - y2 = 0
2xy = 1 .
Substituting y = ½ x into the first equation, we get
x2 -¼ x2 = 0
x2 = ¼ x2
4x4 = 1
Because x is a real number, this equation has two real solutions for x: x = 1/ and x = -1/. Substituting either of these results into the equation 2xy = 1 in turn, we will get the corresponding result for y. Thus, the square roots of i are the numbers 1/ + i/ and -1/ - i/.