Factors of factorials

Number theory

Understanding the relationship between factorials and factors

library(here)
library(tidyverse)
library(knitr)

1 Introduction

In maths, there is a concept called factorial which works like this:

\[ \begin{align} 2!&=2\times1=2\\ 3!&=3\times2\times1=6\\ 4!&=4\times3\times2\times1=24 \end{align} \]

The factors of an integer are the integers that divide equally into it. The factors of \(2!\) are 1 and 2. There are two factors.

\[ \begin{align} \textsf{Factors of 2! (=2):}\\ \quad 1, 2\\ \\ \textsf{Factors of 3! (=6):}\\ \quad 1,2,3,6\\ \\ \textsf{Factors of 4! (=24):}\\ \quad 1,2,3,4,6,8,12,24 \end{align} \]

The relationship between factorials and factors looks like this:

\(n\)	\(n!\)	number of factors of \(n\)
1	1	1
2	2	2
3	6	4
4	24	8

2 Questions

• How many factors has \(5!\) got?
• How many factors has \(6!\) got?
• What pattern do you notice?

3 Analysis

First, I write a function called get_factors() which takes an integer as input and returns a list of the factors of that integer.

# Function to return the list of factors of any number
get_factors <- function(x) {
  if (x <= 0) return(integer(0))
  res <- which(x %% seq_len(x) == 0)
  return(res)
}

I can test the function using one of the integers we already outlined above.

get_factors(6)

[1] 1 2 3 6

Next, I will build a dataframe to view the relationships for a greater set of numbers (from 0 to 10):

factors_of_factorials <- tibble(n = 1:10) |> 
  mutate(
    n_factorial = factorial(n),
    factors_of_n_factorial = map(n_factorial, get_factors),
    total_factors = map_int(factors_of_n_factorial, length)
  )

kable(factors_of_factorials)

n	n_factorial	factors_of_n_factorial	total_factors
1	1	1	1
2	2	1, 2	2
3	6	1, 2, 3, 6	4
4	24	1, 2, 3, 4, 6, 8, 12, 24	8
5	120	1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120	16
6	720	1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 30, 36, 40, 45, 48, 60, 72, 80, 90, 120, 144, 180, 240, 360, 720	30
7	5040	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 28, 30, 35, 36, 40, 42, 45, 48, 56, 60, 63, 70, 72, 80, 84, 90, 105, 112, 120, 126, 140, 144, 168, 180, 210, 240, 252, 280, 315, 336, 360, 420, 504, 560, 630, 720, 840, 1008, 1260, 1680, 2520, 5040	60
8	40320	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 28, 30, 32, 35, 36, 40, 42, 45, 48, 56, 60, 63, 64, 70, 72, 80, 84, 90, 96, 105, 112, 120, 126, 128, 140, 144, 160, 168, 180, 192, 210, 224, 240, 252, 280, 288, 315, 320, 336, 360, 384, 420, 448, 480, 504, 560, 576, 630, 640, 672, 720, 840, 896, 960, 1008, 1120, 1152, 1260, 1344, 1440, 1680, 1920, 2016, 2240, 2520, 2688, 2880, 3360, 4032, 4480, 5040, 5760, 6720, 8064, 10080, 13440, 20160, 40320	96
9	362880	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 27, 28, 30, 32, 35, 36, 40, 42, 45, 48, 54, 56, 60, 63, 64, 70, 72, 80, 81, 84, 90, 96, 105, 108, 112, 120, 126, 128, 135, 140, 144, 160, 162, 168, 180, 189, 192, 210, 216, 224, 240, 252, 270, 280, 288, 315, 320, 324, 336, 360, 378, 384, 405, 420, 432, 448, 480, 504, 540, 560, 567, 576, 630, 640, 648, 672, 720, 756, 810, 840, 864, 896, 945, 960, 1008, 1080, 1120, 1134, 1152, 1260, 1296, 1344, 1440, 1512, 1620, 1680, 1728, 1890, 1920, 2016, 2160, 2240, 2268, 2520, 2592, 2688, 2835, 2880, 3024, 3240, 3360, 3456, 3780, 4032, 4320, 4480, 4536, 5040, 5184, 5670, 5760, 6048, 6480, 6720, 7560, 8064, 8640, 9072, 10080, 10368, 11340, 12096, 12960, 13440, 15120, 17280, 18144, 20160, 22680, 24192, 25920, 30240, 36288, 40320, 45360, 51840, 60480, 72576, 90720, 120960, 181440, 362880	160
10	3628800	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 25, 27, 28, 30, 32, 35, 36, 40, 42, 45, 48, 50, 54, 56, 60, 63, 64, 70, 72, 75, 80, 81, 84, 90, 96, 100, 105, 108, 112, 120, 126, 128, 135, 140, 144, 150, 160, 162, 168, 175, 180, 189, 192, 200, 210, 216, 224, 225, 240, 252, 256, 270, 280, 288, 300, 315, 320, 324, 336, 350, 360, 378, 384, 400, 405, 420, 432, 448, 450, 480, 504, 525, 540, 560, 567, 576, 600, 630, 640, 648, 672, 675, 700, 720, 756, 768, 800, 810, 840, 864, 896, 900, 945, 960, 1008, 1050, 1080, 1120, 1134, 1152, 1200, 1260, 1280, 1296, 1344, 1350, 1400, 1440, 1512, 1575, 1600, 1620, 1680, 1728, 1792, 1800, 1890, 1920, 2016, 2025, 2100, 2160, 2240, 2268, 2304, 2400, 2520, 2592, 2688, 2700, 2800, 2835, 2880, 3024, 3150, 3200, 3240, 3360, 3456, 3600, 3780, 3840, 4032, 4050, 4200, 4320, 4480, 4536, 4725, 4800, 5040, 5184, 5376, 5400, 5600, 5670, 5760, 6048, 6300, 6400, 6480, 6720, 6912, 7200, 7560, 8064, 8100, 8400, 8640, 8960, 9072, 9450, 9600, 10080, 10368, 10800, 11200, 11340, 11520, 12096, 12600, 12960, 13440, 14175, 14400, 15120, 16128, 16200, 16800, 17280, 18144, 18900, 19200, 20160, 20736, 21600, 22400, 22680, 24192, 25200, 25920, 26880, 28350, 28800, 30240, 32400, 33600, 34560, 36288, 37800, 40320, 43200, 44800, 45360, 48384, 50400, 51840, 56700, 57600, 60480, 64800, 67200, 72576, 75600, 80640, 86400, 90720, 100800, 103680, 113400, 120960, 129600, 134400, 145152, 151200, 172800, 181440, 201600, 226800, 241920, 259200, 302400, 362880, 403200, 453600, 518400, 604800, 725760, 907200, 1209600, 1814400, 3628800	270

4 Plots

Finally I can plot the results on a line chart.

factors_of_factorials |> 
  ggplot(aes(x = n, y = total_factors)) +
  geom_line()

5 Conclusion

The line chart shows that the relationship between an integer \(n\) and the number of factors that its factorial has is exponentially increasing.