# How to find inverse of 2 modulo 7 by inspection?

This is from Discrete Mathematics and its Applications

1. By inspection, find an inverse of 2 modulo 7

To do this, I first used Euclid's algorithm to make sure that the greatest common divisor between 2 and 7 is 1. Here is my work for that

7 = 2(3) + 1

2 = 1(2) + 0

Because 1 is the last remainder before the remainder goes to zero, it is the greatest common divisor. Because of 1 is gcd(2, 7) and m, 7, >1, by this theorem

the inverse of a modulo m exists. The inverse of a modulo m is in the form of

a'a $\equiv$ 1 mod(m)

in this case it be

a'*2 $\equiv$ 1 mod(7)

Where a' is the inverse

So from the steps of Euclid's algorithm

1 = (1)(7) + (-3)(2)

(-3)(2) - 1 = (1)(7)

meaning

(-3)(2) $\equiv$ 1 mod (7)

and -3 would be an inverse of 2 modulo 7. How would you find an inverse without going through the steps and just looking at it(by inspection)?

• Look at $\frac{17+1}{2}$. You find the inverse of $2$ modulo $m$ in this way for any odd $m$. – André Nicolas Feb 13 '15 at 4:28
• To me "by inspection" means you don't need to check obvious things like the fact that the gcd of $2$ and $7$ is $1$ ($7$ is prime, after all). The author likely meant for you to think "what number of the form $7n+1$ is a multiple of $2$?" From which you should have quickly seen $n=1$ and with $4$ as the inverse. – Hayden Feb 13 '15 at 4:29
• @Hayden Where did 7n + 1 come from? – committedandroider Feb 13 '15 at 4:30
• @committedandroider If $a\equiv 1 \pmod 7$, then $a=7n+1$ for some $n$. – apnorton Feb 13 '15 at 4:31
• By inspection here just means the numbers are small enough that you can either just "see it" or you can try the small number of possibilities. For example, $2\cdot 4 = 8$, which is $1$ modulo $7$. – aes Feb 13 '15 at 4:35

If you want the multiplicative inverse of $2$ mod $7$, then you want to find an integer $n$ such that $2n = 7k + 1$, where $k$ is a nonnegative integer. Try $k = 1$, because that's the easiest thing to do. Then $2n= 8$, and $n = 4$.

• Although the title mentions 17, the entirety of the question itself uses 7. (Granted, the methodology is pretty much identical) – Hayden Feb 13 '15 at 4:30
• I changed the title, my bad. – committedandroider Feb 13 '15 at 4:31
• Fixed the numbers. +1 – Zubin Mukerjee Feb 13 '15 at 4:36
• So n=4, (4)(2) $\equiv$ 1(mod 7), meaning 8 - 1 is a multiple of 7. How do you guys interpret inverse? What number multiplied by a will with a difference of 1 be a multiple of m? Is that a good way to interpret inverse? I feel like there has to be a better way – committedandroider Feb 13 '15 at 5:11
• @committedandroider A multiplicative inverse of $a$ is a number that you multiply with $a$ to give you the multiplicative identity (which is 1). When you're working with the real numbers, the inverse is $1/a$. With modulo arithmetic, you have to look for numbers congruent to $1$ and find one divisible by $a$. – NoName Feb 13 '15 at 5:30

You could use Euclid's algorithm to compute that gcd(2,7)=1, and from that obtain a solution to $2x+7y=1$, which in turn gives an inverse of $2$ mod $7$.

In this case, Euclid's algorithm terminates very quickly:

$7=2*3+1$

Taking this equation mod $7$ gives:

$2*3+1 \equiv 0 \pmod{7}$

$(-3)*2 \equiv 1 \pmod{7}$

So the inverse of $2$ is $-3$ which is the same as $4$.

• How do you interpret inverse? Like if f(x) = y, f'(y) = x. That makes sense to me. Is there a way to interpret inverse in this case? – committedandroider Feb 13 '15 at 5:12
• Inverse is in the multiplicative sense. That is, the inverse of $a$ is the element $b$ such that $ab=1$. – Dylan Yott Feb 13 '15 at 7:36
• In that sense, -3 would be an inverse of 2 mod 7. 2 mod 7 is actually 2. -3 * 2 gives you -6, not 1. – committedandroider Feb 13 '15 at 23:26
• $-3$ is the inverse of $2$. $4$ is also the inverse of $2$. This is okay because $-3=4$ in the ring $\mathbb Z / 7 \mathbb Z$ – Dylan Yott Feb 14 '15 at 18:13

If the modulus $\,m\equiv \pm1 \pmod{a},\,$ then we can easily invert $\,a\pmod m\,$ as follows

$(1)\qquad\quad {\rm mod}\,\ m = na\!-\!1\!:\ \ \ na\, \equiv 1\ \Rightarrow\ a^{-1}\equiv\,\ n \,=\, \color{#c00}{(1\!+\!m)/a}$

$(2)\qquad\quad {\rm mod}\,\ m = na\!+\!1\!:\, -na\equiv 1\:\Rightarrow\ a^{-1}\equiv -n = \color{#0a0}{(1\!-\!m)/a}$

E.g. your $\,m = 7\equiv \pm1\pmod{2},\,$ hence by $\,(2),\ \ 2^{-1} \equiv \color{#0a0}{(1\!-\!7)/2} \equiv -3$

Alternatively we can apply the case $(1)$ obtaining $\,2^{-1} \equiv \color{#c00}{(1\!+\!7)/2}\equiv\ 4$

This can be viewed as an optimization of the Extended Euclidean algorithm in the case that it terminates in a single step (or ditto for Gauss's method for modular inversion).

• What would n be in the case of inverse of 2 modulo 7? can you show a test run of that? – committedandroider Feb 13 '15 at 5:20
• @committedandroider Observe that $4 \cdot 2 \equiv 8 \equiv 1 \pmod{7}$, so $2^{-1} \equiv 4 \pmod{7}$. Also, observe that $3 \cdot 2 \equiv 6 \equiv -1 \pmod{7}$, so $2^{-1} \equiv -3 \equiv 4 \pmod{7}$. – N. F. Taussig Feb 13 '15 at 12:44
• @com I explained this in an edit to the answer. – Bill Dubuque Feb 13 '15 at 19:32
 1 = 8 (mod 7) and 8 is a multiple of 2.

2 x 4 = 8 = 1 mod 7


So $2^{-1} \equiv 4 \pmod 7$

Something harder. Find $5^{-1} \pmod 7$

1 = 8 = 15 mod 7 and 15 is a multiple of 5

$5 \cdot 3 = 15 = 1\pmod 7$

So $5^{-1} = 3 \pmod 7$