Efficient Algorithm for Exponentiation of Imaginary Number

Quickest way to find 'i' without calculator

Hassan Raza Khan¹

(¹) Hassan Raza Khan is a high-school student at The City School DHA Campus, Lahore.

1 Introduction

Say you want to find the value of i) ${\textstyle i^{8237918273}}$; ii) ${\textstyle i^{823718932}}$; iii) ${\textstyle i^{1291393767}}$; iv) ${\textstyle i^{812731274}}$;

and there's like 20 seconds left in the exam, and this paper does not allow calculators. You start solving it something like this:

$i^{8237918273}={\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\cdot {\sqrt {-1}}\ldots$

and before you know it, the 20 seconds are over. You couldn't even answer any of the question.

The student next to you, the class topper, recognizes that we can find the value of such a large number of ${\textstyle i}$ using polar coordinates, by employing Euler's formula. He starts writing:

$e^{i\theta }=\cos(\theta )+i\sin(\theta {).}$

$i=\cos \left({\frac {\pi }{2}}\right)+i\sin \left({\frac {\pi }{2}}\right)$

$i^{n}=\left(\cos \left({\frac {\pi }{2}}\right)+i\sin \left({\frac {\pi }{2}}\right)\right)^{n}$

He now starts using De Moivre's Theorem ${\textstyle n=8237918273}$.

$i^{8237918273}=\cos \left(8237918273\times {\frac {\pi }{2}}\right)+i\sin \left(8237918273\times {\frac {\pi }{2}}\right)$

By now, the proctor comes and snatches his paper. He could not even solve one of the 4 parts. None of the students was able to solve any part within less than 20 seconds– except a boy named Draco, who solved all 4 of these parts within less than 20 seconds. Fun fact: he didn't even do any calculations on the paper. He only used the last two digits and performed The Algorithm in his mind.

2 The Algorithm

The algorithm involves three steps:

1. Real/Imaginary number determination If the last two digits are even, the value remains unchanged (either 1 or -1). If odd, subtract 1 (resulting in either i or -i).
2. Division by 2 Divide the resultant number by 2.
3. Sign determination If the remaining number after division by 2 is odd, the sign is minus; if it's even, the sign is plus.

3 Proving that only the last two digits are necessary

To demonstrate that only the last two digits are necessary for determining the value of ${\textstyle i^{n}}$, where ${\textstyle n}$ is a large exponent, we can employ the periodicity of powers of ${\textstyle i}$.

Consider the powers of ${\textstyle i}$ when raised to successive positive integer exponents:

${\begin{aligned}i^{1}&=i\\i^{2}&=-1\\i^{3}&=-i\\i^{4}&=1\\i^{5}&=i\\i^{6}&=-1\\i^{7}&=-i\\i^{8}&=1\\\vdots \end{aligned}}$

We observe that the powers of ${\textstyle i}$ repeat in cycles of four: ${\textstyle i,-1,-i,1}$. This periodicity implies that the value of ${\textstyle i^{n}}$ depends only on the remainder when ${\textstyle n}$ is divided by 4.

Now, let's consider the exponent ${\textstyle n=8237918273}$. We want to find ${\textstyle i^{n}}$:

$i^{8237918273}=i^{4k+r},$

Consider ${\textstyle i^{8237918273}}$. We can express this as ${\textstyle i^{4k+r}}$, where ${\textstyle k}$ is an integer and ${\textstyle r}$ is the remainder when ${\textstyle 8237918273}$ is divided by ${\textstyle 4}$. In this case, ${\textstyle r=73}$.

Now, let's break down ${\textstyle i^{8237918273}}$ further:

$i^{8237918273}=i^{8237918200+73}.$

Since ${\textstyle 8237918200}$ has two zeros and is divisible by 100, and since 100 is divisible by 4, we can conclude that ${\textstyle 8237918200}$ is divisible by 4.

Therefore, ${\textstyle i^{4k+r}=i^{8237918200+73}=i^{8237918200}\cdot i^{73}.}$

Now, ${\textstyle i^{8237918200}}$ simplifies to ${\textstyle 1}$ since any power of ${\textstyle i}$ with an exponent divisible by 4 is equal to 1. Therefore, we have:

$1\cdot i^{73}=i^{73}.$

So, essentially, ${\textstyle i^{8237918273}}$ is congruent to ${\textstyle i^{73}}$. This result demonstrates a general pattern: for any large exponent ${\textstyle n}$, ${\textstyle i^{n}}$ is congruent to ${\textstyle i^{r}}$, where ${\textstyle r}$ is the remainder when ${\textstyle n}$ is divided by 4. Thus, only the last two digits of the exponent (${\textstyle 73}$ in this case) are crucial for determining the value of ${\textstyle i^{n}}$.

This observation allows for a more efficient approach when faced with large exponentiation of ${\textstyle i}$ without the need to calculate the entire exponent. The periodic nature of ${\textstyle i}$ simplifies the computation, making it feasible to focus solely on the last two digits of the exponent.

4 Demonstrating the algorithm

What Draco essentially did was use the above three-step algorithm like so:

1. ${\textstyle i^{8237918273}}$;
   1. Real/Imaginary number determination The last two digits are 73, an odd number. So it must be an imaginary number (either ${\textstyle i}$ or ${\textstyle -i}$). Since the number is odd, we subtract it by 1 to make it parallel to its even counterpart, resulting in 72.
   2. Division by 2 Dividing 72 by 2 results in 36, an even number.
   3. Sign determination Since the resultant number after division by 2 is even, we can establish that the sign of this number would be positive. Since the number is both imaginary and positive, it must be ${\textstyle i}$.
2. ${\textstyle i^{823718932}}$;
   1. Real/Imaginary number determination The last two digits are 32, an even number. So it must be a real number (either 1 or -1). Since the number is even, we do not subtract it, so it remains 32.
   2. Division by 2 Dividing 32 by 2 results in 16, an even number.
   3. Sign determination Since the resultant number after division by 2 is even, we can establish that the sign of this number would be positive. Since the number is both real and positive, it must be 1.
3. ${\textstyle i^{1291393767}}$;
   1. Real/Imaginary number determination The last two digits are 67, an odd number. So it must be an imaginary number (either ${\textstyle i}$ or ${\textstyle -i}$). Since the number is odd, we subtract it by 1 to make it parallel to its even counterpart, resulting in 66.
   2. Division by 2 Dividing 66 by 2 results in 33, an odd number.
   3. Sign determination Since the resultant number after division by 2 is odd, we can establish that the sign of this number would be negative. Since the number is both imaginary and negative, it must be ${\textstyle -i}$.
4. ${\textstyle i^{812731274}}$;
   1. Real/Imaginary number determination The last two digits are 74, an even number. So it must be a real number (either 1 or -1). Since the number is even, we do not subtract it, so it remains 74.
   2. Division by 2 Dividing 74 by 2 results in 37, an odd number.
   3. Sign determination Since the resultant number after division by 2 is odd, we can establish that the sign of this number would be negative. Since the number is both real and negative, it must be ${\textstyle -1}$.

In Table 1, we present a matrix illustrating the relationship between the final and initial numbers corresponding to different values of the complex unit ${\textstyle i}$. The table categorizes the initial and final numbers as either even or odd and displays the resulting values of ${\textstyle i}$ for each combination. This matrix provides a concise reference for understanding the periodicity of ${\textstyle i}$ and its cyclic behavior based on the evenness or oddness of the exponents. Memorizing this table can make you the Draco in the same circumstance.

Table. 1 Matrix of Initial and Final Numbers Corresponding to Values of $i$

		Initial Number
		Even	Odd
Final Number	Even	1	i
	Odd	-1	-i

5 Purpose of Odd/Even Analysis

The divisional remainder and its congruent complex numeber counterpart can be illustrated as below:

$1\equiv i{\pmod {4}}$ $2\equiv -1{\pmod {4}}$ $3\equiv -i{\pmod {4}}$ $0\equiv 1{\pmod {4}}$

The odd/even analysis serves as a strategic approach to efficiently determine the divisional remainder of the numbers in the context of complex exponentiation. By focusing solely on the last two digits of the exponent, we can employ a systematic method to evaluate whether the number is divisible by 4.

For instance, consider an exponent with the last two digits being 32. If we divide 32 by 2, we obtain 16, an even number. This indicates that the original exponent is divisible by 2 twice, meaning it is divisible by 4. Therefore, the number is congruent to 0 modulo 4.

On the contrary, let's take another example with the last two digits being 34. If we divide 34 by 2, we get 17, an odd number. In this case, 34 can only be divided by 2 once while remaining a whole number. Consequently, it is congruent to 2 modulo 4.

The same approach is applied to odd numbers, where they are subtracted by 1 in order to align them with their even counterparts. This adjustment ensures that odd numbers maintain their distinctive properties in the modulo 4 arithmetic.

It's worth noting that one could also add 1 instead of subtracting 1 from the odd number. However, by adopting the subtraction approach, the signs become universally positive for even final numbers and negative for odd final numbers. This consistency simplifies the association of odd numbers with negative results and even numbers with positive results, aiding in a more straightforward interpretation of the outcomes.

In summary, the odd/even analysis not only streamlines the computation of complex exponentiation but also provides a clear and consistent method for determining the divisional remainder based on the last two digits of the exponent.

Acknowledgements

I would like to express my gratitude for the support and encouragement I received from my friends, family, and teachers throughout the course of this research paper.

Lastly, I owe a special debt of gratitude to Dr Javed Hussain and Dr Zarqa Bano from IBA Sukkur University for their expert guidance. As a high school student new to research papers, their assistance was invaluable. They not only helped me with the paper but also instilled in me the importance of intellectual curiosity and hard work.

BIBLIOGRAPHY

Khan, H. R. (2023, October 27). Efficient Algorithm for Identifying Repeating Patterns Modulo 4 and its Application in Complex Numbers. https://doi.org/10.31219/osf.io/8zc7e

Euler, L. (1748). Introductio in analysin infinitorum. Apud Marcum-Michaelem Bousquet & Socios.

De Moivre, A. (1730). Miscellanea Analytica de Seriebus et Quadraturis [Analytical Miscellany on Series and Integration].

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Halmos, P. (1980). The heart of mathematics. American Mathematical Monthly, 87, 519–524.

Nahin, P. (1998). An Imaginary Tale: The Story of ${\sqrt {-1}}$. New Jersey: Princeton University Press.