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«i (number)» redirects here. For internet numbers, see i-number.

i in the complex or Cartesian plane. Real numbers lie on the horizontal axis, and imaginary numbers lie on the vertical axis.

The imaginary unit or unit imaginary number (i) is a solution to the quadratic equation ${displaystyle x^{2}+1=0}$ . Although there is no real number with this property, i can be used to extend the real numbers to what are called complex numbers, using addition and multiplication. A simple example of the use of i in a complex number is .

Imaginary numbers are an important mathematical concept; they extend the real number system to the complex number system , in which at least one root for every nonconstant polynomial exists (see Algebraic closure and Fundamental theorem of algebra). Here, the term «imaginary» is used because there is no real number having a negative square.

There are two complex square roots of −1: i and , just as there are two complex square roots of every real number other than zero (which has one double square root).

In contexts in which use of the letter i is ambiguous or problematic, the letter j is sometimes used instead. For example, in electrical engineering and control systems engineering, the imaginary unit is normally denoted by j instead of i, because i is commonly used to denote electric current.^[1]

Definition[edit]

The powers of i return cyclic values:
(repeats the pattern from bold blue area)
${displaystyle i^{-3}=i}$
${displaystyle i^{-2}=-1}$
${displaystyle i^{-1}=-i}$
${displaystyle i^{0}=1}$
${displaystyle i^{1}=i}$
$i^{{2}}=-1$
${displaystyle i^{3}=-i}$
${displaystyle i^{4}=1}$
${displaystyle i^{5}=i}$
${displaystyle i^{6}=-1}$
(repeats the pattern from bold blue area)

The imaginary number i is defined solely by the property that its square is −1:

${displaystyle i^{2}=-1.}$

With i defined this way, it follows directly from algebra that i and 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 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 $i^{2}$ with −1). Higher integral powers of i can also be replaced with , 1, i, or −1:

${displaystyle i^{3}=i^{2}i=(-1)i=-i}$

${displaystyle i^{4}=i^{3}i=(-i)i=-(i^{2})=-(-1)=1}$

or, equivalently,

${displaystyle i^{4}=(i^{2})(i^{2})=(-1)(-1)=1}$

${displaystyle i^{5}=i^{4}i=(1)i=i}$

Similarly, as with any non-zero real number:

${displaystyle i^{0}=i^{1-1}=i^{1}i^{-1}=i^{1}{frac {1}{i}}=i{frac {1}{i}}={frac {i}{i}}=1}$

As a complex number, i can be represented in rectangular form as , with a zero real component and a unit imaginary component. In polar form, i can be represented as ${displaystyle 1times e^{ipi /2}}$ (or just ${displaystyle e^{ipi /2}}$ ), with an absolute value (or magnitude) of 1 and an argument (or angle) of ${displaystyle {tfrac {pi }{2}}}$ radians. (Adding any multiple of 2π to this angle works as well.) In the complex plane (also known as the Argand plane), which is a special interpretation of a Cartesian plane, i is the point located one unit from the origin along the imaginary axis (which is orthogonal to the real axis).

i vs. −i[edit]

Being a quadratic polynomial with no multiple root, the defining equation ${displaystyle x^{2}=-1}$ 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 , 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 .^[2] After all, although and are not quantitatively equivalent (they are negatives of each other), there is no algebraic difference between and , 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 replacing every occurrence of (and, therefore, every occurrence of replaced by ), all facts and theorems would remain valid. The distinction between the two roots x of ${displaystyle x^{2}+1=0}$ , 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».^[3]

The issue can be a subtle one. One way of articulating the situation is that although the complex field is unique (as an extension of the real numbers) up to isomorphism, it is not unique up to a unique isomorphism. Indeed, there are two field automorphisms of C that keep each real number fixed, namely the identity and complex conjugation. For more on this general phenomenon, see Galois group.

Matrices[edit]

Some imaginary units correspond to points ( x, y ) on the hyperbola xy = −1.

Using the concepts of matrices and matrix multiplication, imaginary units can be represented in linear algebra. The value of 1 is represented by an identity matrix I and the value of i is represented by any matrix J satisfying J² = −I. A typical choice is

${displaystyle I={begin{pmatrix}1&0\0&1end{pmatrix}},qquad J={begin{pmatrix}0&-1\1&0end{pmatrix}},.}$

More generally, a real-valued 2 × 2 matrix J satisfies J² = −I if and only if J has a matrix trace of zero and a matrix determinant of one, so J can be chosen to be

${displaystyle J={begin{pmatrix}z&x\y&-zend{pmatrix}},,}$

whenever −z² − xy = 1. The product xy is negative because xy = −(1 + z²); thus, the points (x, y) lie on hyperbolas determined by z in quadrant II or IV.

The usual rules of complex number mathematics work with these matrices because I × I = I, I × J = J, J × I = J, and J × J = −I. For example,

${displaystyle {begin{aligned}J^{-1}&=-J,,\left(aI+bJright)+left(cI+dJright)&=(a+c)I+(b+d)J,,\left(aI+bJright)times left(cI+dJright)&=(ac-bd)I+(ad+bc)J,.end{aligned}}}$

Proper use[edit]

The imaginary unit is sometimes written in advanced mathematics contexts^[2] (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 , 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:^[4]

{displaystyle -1=icdot i={sqrt {-1}}cdot {sqrt {-1}}={sqrt {(-1)cdot (-1)}}={sqrt {1}}=1qquad {text{(incorrect).}}}

Similarly:

${displaystyle {frac {1}{i}}={frac {sqrt {1}}{sqrt {-1}}}={sqrt {frac {1}{-1}}}={sqrt {frac {-1}{1}}}={sqrt {-1}}=iqquad {text{(incorrect).}}}$

Generally, the calculation rules

and

${displaystyle {frac {sqrt {a}}{sqrt {b}}}={sqrt {frac {a}{b}}}}$

are guaranteed to be valid for real, positive values of a and b only.^[5]^[6]^[7]

When a or b is real but negative, these problems can be avoided by writing and manipulating expressions like , rather than . For a more thorough discussion, see square root and branch point.

Properties[edit]

Square roots[edit]

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^[a]

${displaystyle pm left({frac {sqrt {2}}{2}}+{frac {sqrt {2}}{2}}iright)=pm {frac {sqrt {2}}{2}}(1+i).}$

Indeed, squaring both expressions yields:

${displaystyle {begin{aligned}left(pm {frac {sqrt {2}}{2}}(1+i)right)^{2} &=left(pm {frac {sqrt {2}}{2}}right)^{2}(1+i)^{2} \&={frac {1}{2}}(1+2i+i^{2})\&={frac {1}{2}}(1+2i-1) \&=i.end{aligned}}}$

Using the radical sign for the principal square root, we get:

${displaystyle {sqrt {i}}={frac {sqrt {2}}{2}}(1+i).}$

Cube roots[edit]

The three cube roots of i are:^[9]

${displaystyle {frac {sqrt {3}}{2}}+{frac {i}{2}},}$

and

${displaystyle -{frac {sqrt {3}}{2}}+{frac {i}{2}}.}$

Similar to all the roots of 1, all the roots of i are the vertices of regular polygons, which are inscribed within the unit circle in the complex plane.

Multiplication and division[edit]

Multiplying a complex number by i gives:

${displaystyle i(a+bi)=ai+bi^{2}=-b+ai.}$

(This is equivalent to a 90° counter-clockwise rotation of a vector about the origin in the complex plane.)

Dividing by i is equivalent to multiplying by the reciprocal of i:

${displaystyle {frac {1}{i}}={frac {1}{i}}cdot {frac {i}{i}}={frac {i}{i^{2}}}={frac {i}{-1}}=-i~.}$

Using this identity to generalize division by i to all complex numbers gives:

${displaystyle {frac {a+bi}{i}}=-i(a+bi)=-ai-bi^{2}=b-ai.}$

(This is equivalent to a 90° clockwise rotation of a vector about the origin in the complex plane.)

Powers[edit]

The powers of i repeat in a cycle expressible with the following pattern, where n is any integer:

${displaystyle i^{4n}=1}$

${displaystyle i^{4n+1}=i}$

${displaystyle i^{4n+2}=-1}$

${displaystyle i^{4n+3}=-i,}$

This leads to the conclusion that

${displaystyle i^{n}=i^{(n{bmod {4}})}}$

where mod represents the modulo operation. Equivalently:

${displaystyle i^{n}=cos(npi /2)+isin(npi /2)}$

Although we do not give the details here, if one chooses branch cuts and principal values to support it then this last equation would apply to all complex values of n.^{[citation needed]}

i raised to the power of i[edit]

Making use of Euler’s formula, ${displaystyle i^{i}}$ has infinitely many values

${displaystyle i^{i}=left(e^{i(pi /2+2kpi )}right)^{i}=e^{i^{2}(pi /2+2kpi )}=e^{-(pi /2+2kpi )},,}$

for any integer k. A common principal value corresponds to k=0 and gives ${displaystyle i^{i}=e^{-pi /2}}$ , which is 0.207879576….^[10]^[11]

Factorial[edit]

The factorial of the imaginary unit i is most often given in terms of the gamma function evaluated at :^[12]^[13]^[14]

{displaystyle i!=Gamma (1+i)=iGamma (i)approx 0.498015668-0.154949828i.}

^[15]

The magnitude of this number is

${displaystyle |Gamma (1+i)|={sqrt {frac {pi }{sinh pi }}}=0.521564046ldots ,}$

^[16]

while its argument is

${displaystyle arg {Gamma (1+i)}=lim _{nto infty }{biggl (}ln {n}-sum _{k=1}^{n}operatorname {arccot} {k}{biggr )}approx -0.301640320.}$

^[17]

Other operations[edit]

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. The following functions are well-defined, single-valued functions when x is a positive real number.

A number raised to the ni power is:

${displaystyle x^{ni}=cos(nln x)+isin(nln x).}$

The ni^th root of a number is:

${displaystyle {sqrt[{ni}]{x}}=cos left({frac {ln x}{n}}right)-isin left({frac {ln x}{n}}right)~.}$

The cosine of ni is:

${displaystyle cos ni=cosh n={frac {1}{2}}left(e^{n}+{frac {1}{e^{n}}}right)={frac {e^{2n}+1}{2e^{n}}},,}$

which is a real number when n is a real number.

The sine of ni is:

${displaystyle sin ni=isinh n={frac {1}{2}}left(e^{n}-{frac {1}{e^{n}}}right)i={frac {e^{2n}-1}{2e^{n}}}i,,}$

which is a purely imaginary number when n is a real number.

In contrast, many functions involving i, including those that depend upon log i or the logarithm of another complex number, are complex multi-valued functions, with different values on different branches of the Riemann surface the function is defined on.^[18] For example, if one chooses any branch where log i = πi/2 then one can write

${displaystyle log _{i}x=-{frac {2iln x}{pi }},,}$

when x is a positive real number. When x is not a positive real number in the above formulas then one must precisely specify the branch to get a single-valued function; see complex logarithm.

History[edit]

Designating square roots of negative numbers as «imaginary» is generally credited to René Descartes, and Isaac Newton used the term as early as 1670.^[19]^[20] The i notation was introduced by Leonhard Euler.^[21]

Notes[edit]

^ To find such a number, one can solve the equation ${textstyle (x+iy)^{2}=i}$ where x and y are real parameters to be determined, or equivalently ${displaystyle x^{2}+2ixy-y^{2}=i.}$ Because the real and imaginary parts are always separate, we regroup the terms, ${displaystyle x^{2}-y^{2}+2ixy=0+i.}$ By equating coefficients, separating the real part and imaginary part, we get a system of two equations:

${displaystyle {begin{aligned}x^{2}-y^{2}&=0\[3mu]2xy&=1.end{aligned}}}$

Substituting ${displaystyle y={tfrac {1}{2}}x^{-1}}$ into the first equation, we get ${displaystyle x^{2}-{tfrac {1}{4}}x^{-2}=0}$ ${displaystyle implies 4x^{4}=1.}$ Because x is a real number, this equation has two real solutions for x: ${displaystyle x={tfrac {1}{sqrt {2}}}}$ and ${displaystyle x=-{tfrac {1}{sqrt {2}}}}$ . Substituting either of these results into the equation in turn, we will get the corresponding result for y. Thus, the square roots of i are the numbers ${displaystyle {tfrac {1}{sqrt {2}}}+{tfrac {1}{sqrt {2}}}i}$ and ${displaystyle -{tfrac {1}{sqrt {2}}}-{tfrac {1}{sqrt {2}}}i}$ .^[8]

References[edit]

^ Boas, Mary L. (2006). Mathematical Methods in the Physical Sciences (3rd ed.). New York [u.a.]: Wiley. p. 49. ISBN 0-471-19826-9.
^ ^a ^b Weisstein, Eric W. «Imaginary Unit». mathworld.wolfram.com. Retrieved 10 August 2020.
^ Doxiadēs, Apostolos K.; Mazur, Barry (2012). Circles Disturbed: The interplay of mathematics and narrative (illustrated ed.). Princeton University Press. p. 225. ISBN 978-0-691-14904-2 – via Google Books.
^ Bunch, Bryan (2012). Mathematical Fallacies and Paradoxes (illustrated ed.). Courier Corporation. p. 31-34. ISBN 978-0-486-13793-3 – via Google Books.
^ Kramer, Arthur (2012). Math for Electricity & Electronics (4th ed.). Cengage Learning. p. 81. ISBN 978-1-133-70753-0 – via Google Books.
^ Picciotto, Henri; Wah, Anita (1994). Algebra: Themes, tools, concepts (Teachers’ ed.). Henri Picciotto. p. 424. ISBN 978-1-56107-252-1 – via Google Books.
^ Nahin, Paul J. (2010). An Imaginary Tale: The story of «i» [the square root of minus one]. Princeton University Press. p. 12. ISBN 978-1-4008-3029-9 – via Google Books.
^ «What is the square root of i ?». University of Toronto Mathematics Network. Retrieved 26 March 2007.
^ Zill, Dennis G.; Shanahan, Patrick D. (2003). A first course in complex analysis with applications. Boston: Jones and Bartlett. pp. 24–25. ISBN 0-7637-1437-2. OCLC 50495529.
^ Wells, David (1997) [1986]. The Penguin Dictionary of Curious and Interesting Numbers (revised ed.). UK: Penguin Books. p. 26. ISBN 0-14-026149-4.
^ Sloane, N. J. A. (ed.). «Sequence A049006 (Decimal expansion of i^i = exp(-Pi/2))». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
^ Sloane, N. J. A. (ed.). «Sequence A212879 (Decimal expansion of the absolute value of i!)». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
^ Ivan, M.; Thornber, N.; Kouba, O.; Constales, D. (2013). «Arggh! Eye factorial . . . Arg(i!)». American Mathematical Monthly. 120: 662–665. doi:10.4169/amer.math.monthly.120.07.660. S2CID 24405635.
^ Finch, S. (3 November 2022). «Errata and Addenda to Mathematical Constants«. arXiv:2001.00578 [math.HO].
^ Sloane, N. J. A. (ed.). «Sequence A212877 (Decimal expansion of the real part of i!)». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.Sloane, N. J. A. (ed.). «Sequence A212878 (Decimal expansion of the negated imaginary part of i!)». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
^ Sloane, N. J. A. (ed.). «Sequence A212879 (Decimal expansion of the absolute value of i!)». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
^ Sloane, N. J. A. (ed.). «Sequence A212880 (Decimal expansion of the negated argument of i!)». The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
^ Gbur, Greg (2011). Mathematical Methods for Optical Physics and Engineering. Cambridge, U.K.: Cambridge University Press. pp. 278–284. ISBN 978-0-511-91510-9. OCLC 704518582.
^ Silver, Daniel S. (November–December 2017). «The New Language of Mathematics». American Scientist. 105 (6): 364–371. doi:10.1511/2017.105.6.364.
^ «imaginary number». Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
^ Boyer, Carl B.; Merzbach, Uta C. (1991). A History of Mathematics. John Wiley & Sons. pp. 439–445. ISBN 978-0-471-54397-8.

External links[edit]

Euler, Leonhard. «Imaginary Roots of Polynomials». at «Convergence». mathdl.maa.org. Mathematical Association of America. Archived from the original on 13 July 2007.

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