Narcissistic number

In number theory, a narcissistic number^[1]^[2] (also known as a pluperfect digital invariant (PPDI),^[3] an Armstrong number^[4] (after Michael F. Armstrong)^[5] or a plus perfect number)^[6] in a given number base $b$ is a number that is the sum of its own digits each raised to the power of the number of digits.

Definition

Let $n$ be a natural number. We define the narcissistic function for base $b>1$ $F_{b}:\mathbb {N} \rightarrow \mathbb {N}$ to be the following:

F_{b}(n)=\sum _{i=0}^{k-1}d_{i}^{k}.

where $k=\lfloor \log _{b}{n}\rfloor +1$ is the number of digits in the number in base $b$ , and

d_{i}={\frac {n{\bmod {b^{i+1}}}-n{\bmod {b}}^{i}}{b^{i}}}

is the value of each digit of the number. A natural number $n$ is a narcissistic number if it is a fixed point for $F_{b}$ , which occurs if $F_{b}(n)=n$ . The natural numbers $0\leq n<b$ are trivial narcissistic numbers for all $b$ , all other narcissistic numbers are nontrivial narcissistic numbers.

For example, the number 153 in base $b=10$ is a narcissistic number, because $k=3$ and $153=1^{3}+5^{3}+3^{3}$ .

A natural number $n$ is a sociable narcissistic number if it is a periodic point for $F_{b}$ , where $F_{b}^{p}(n)=n$ for a positive integer $p$ (here $F_{b}^{p}$ is the $p$ th iterate of $F_{b}$ ), and forms a cycle of period $p$ . A narcissistic number is a sociable narcissistic number with $p=1$ , and an amicable narcissistic number is a sociable narcissistic number with $p=2$ .

All natural numbers $n$ are preperiodic points for $F_{b}$ , regardless of the base. This is because for any given digit count $k$ , the minimum possible value of $n$ is $b^{k-1}$ , the maximum possible value of $n$ is $b^{k}-1\leq b^{k}$ , and the narcissistic function value is $F_{b}(n)=k(b-1)^{k}$ . Thus, any narcissistic number must satisfy the inequality $b^{k-1}\leq k(b-1)^{k}\leq b^{k}$ . Multiplying all sides by ${\frac {b}{(b-1)^{k}}}$ , we get ${\left({\frac {b}{b-1}}\right)}^{k}\leq bk\leq b{\left({\frac {b}{b-1}}\right)}^{k}$ , or equivalently, $k\leq {\left({\frac {b}{b-1}}\right)}^{k}\leq bk$ . Since ${\frac {b}{b-1}}\geq 1$ , this means that there will be a maximum value $k$ where ${\left({\frac {b}{b-1}}\right)}^{k}\leq bk$ , because of the exponential nature of ${\left({\frac {b}{b-1}}\right)}^{k}$ and the linearity of $bk$ . Beyond this value $k$ , $F_{b}(n)\leq n$ always. Thus, there are a finite number of narcissistic numbers, and any natural number is guaranteed to reach a periodic point or a fixed point less than $b^{k}-1$ , making it a preperiodic point. Setting $b$ equal to 10 shows that the largest narcissistic number in base 10 must be less than $10^{60}$ .^[1]

The number of iterations $i$ needed for $F_{b}^{i}(n)$ to reach a fixed point is the narcissistic function's persistence of $n$ , and undefined if it never reaches a fixed point.

A base $b$ has at least one two-digit narcissistic number if and only if $b^{2}+1$ is not prime, and the number of two-digit narcissistic numbers in base $b$ equals $\tau (b^{2}+1)-2$ , where $\tau (n)$ is the number of positive divisors of $n$ .

Every base $b\geq 3$ that is not a multiple of nine has at least one three-digit narcissistic number. The bases that do not are

2, 72, 90, 108, 153, 270, 423, 450, 531, 558, 630, 648, 738, 1044, 1098, 1125, 1224, 1242, 1287, 1440, 1503, 1566, 1611, 1620, 1800, 1935, ... (sequence A248970 in the OEIS)

There are only 88 narcissistic numbers in base 10, of which the largest is

115,132,219,018,763,992,565,095,597,973,971,522,401

with 39 digits.^[1]

Narcissistic numbers and cycles of F_b for specific b

All numbers are represented in base $b$ . '#' is the length of each known finite sequence.

$b$	Narcissistic numbers	#	Cycles	OEIS sequence(s)
2	0, 1	2	$\varnothing$
3	0, 1, 2, 12, 22, 122	6	$\varnothing$
4	0, 1, 2, 3, 130, 131, 203, 223, 313, 332, 1103, 3303	12	$\varnothing$	A010344 and A010343
5	0, 1, 2, 3, 4, 23, 33, 103, 433, 2124, 2403, 3134, 124030, 124031, 242423, 434434444, ...	18	1234 → 2404 → 4103 → 2323 → 1234 3424 → 4414 → 11034 → 20034 → 20144 → 31311 → 3424 1044302 → 2110314 → 1044302 1043300 → 1131014 → 1043300	A010346
6	0, 1, 2, 3, 4, 5, 243, 514, 14340, 14341, 14432, 23520, 23521, 44405, 435152, 5435254, 12222215, 555435035 ...	31	44 → 52 → 45 → 105 → 330 → 130 → 44 13345 → 33244 → 15514 → 53404 → 41024 → 13345 14523 → 32253 → 25003 → 23424 → 14523 2245352 → 3431045 → 2245352 12444435 → 22045351 → 30145020 → 13531231 → 12444435 115531430 → 230104215 → 115531430 225435342 → 235501040 → 225435342	A010348
7	0, 1, 2, 3, 4, 5, 6, 13, 34, 44, 63, 250, 251, 305, 505, 12205, 12252, 13350, 13351, 15124, 36034, 205145, 1424553, 1433554, 3126542, 4355653, 6515652, 125543055, ...	60		A010350
8	0, 1, 2, 3, 4, 5, 6, 7, 24, 64, 134, 205, 463, 660, 661, 40663, 42710, 42711, 60007, 62047, 636703, 3352072, 3352272, ...	63		A010354 and A010351
9	0, 1, 2, 3, 4, 5, 6, 7, 8, 45, 55, 150, 151, 570, 571, 2446, 12036, 12336, 14462, 2225764, 6275850, 6275851, 12742452, ...	59		A010353
10	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407, 1634, 8208, 9474, 54748, 92727, 93084, 548834, ...	88		A005188
11	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, 56, 66, 105, 307, 708, 966, A06, A64, 8009, 11720, 11721, 12470, ...	135		A0161948
12	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, 25, A5, 577, 668, A83, 14765, 938A4, 369862, A2394A, ...	88		A161949
13	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, 14, 36, 67, 77, A6, C4, 490, 491, 509, B85, 3964, 22593, 5B350, ...	202		A0161950
14	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, 136, 409, 74AB5, 153A632, ...	103		A0161951
15	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, 78, 88, C3A, D87, 1774, E819, E829, 7995C, 829BB, A36BC, ...	203		A0161952
16	0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 156, 173, 208, 248, 285, 4A5, 5B0, 5B1, 60B, 64B, 8C0, 8C1, 99A, AA9, AC3, CA8, E69, EA0, EA1, B8D2, 13579, 2B702, 2B722, 5A07C, 5A47C, C00E0, C00E1, C04E0, C04E1, C60E7, C64E7, C80E0, C80E1, C84E0, C84E1, ...	294		A161953

Extension to negative integers

Narcissistic numbers can be extended to the negative integers by use of a signed-digit representation to represent each integer.

Programming example

Python

The example below implements the narcissistic function described in the definition above to search for narcissistic functions and cycles in Python.

def ppdif(x, b):
    y = x
    digit_count = 0
    while y > 0:
        digit_count = digit_count + 1
        y = y // b
    total = 0
    while x > 0:
        total = total + pow(x % b, digit_count)
        x = x // b
    return total

def ppdif_cycle(x, b):
    seen = []
    while x not in seen:
        seen.append(x)
        x = ppdif(x, b)
    cycle = []
    while x not in cycle:
        cycle.append(x)
        x = ppdif(x, b)
    return cycle

The following Python program determines whether the integer entered is a Narcissistic / Armstrong number or not.

def no_of_digits(num):
    i = 0
    while num > 0:
        num //= 10
        i+=1

    return i

def required_sum(num):
    i = no_of_digits(num)
    s = 0
    
    while num > 0:
        digit = num % 10
        num //= 10
        s += pow(digit, i)
        
    return s

num = int(input("Enter number:"))
s = required_sum(num)
     
if s == num:
    print("Armstrong Number")
else:
    print("Not Armstrong Number")

Java

The following Java program determines whether the integer entered is a Narcissistic / Armstrong number or not.

import java.util.Scanner;

public class ArmstrongNumber {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter a positive integer: ");
        int number = in.nextInt();

        if (isArmstrongNumber(number)) {
            System.out.println(number + " is an Armstrong number.");
        } else {
            System.out.println(number + " is not an Armstrong number.");
        }
    }

    public static boolean isArmstrongNumber(int number) {
        int sum = 0;
        String numberString = Integer.toString(number);
        int numberOfDigits = numberString.length();

        for (int i = 0; i < numberOfDigits; i++) {
            int digit = Character.getNumericValue(numberString.charAt(i));
            sum += Math.pow(digit, numberOfDigits);
        }

        return sum == number;
    }
}

C#

The following C# program determines whether the integer entered is a Narcissistic / Armstrong number or not.

using System;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Enter the number:");
        int value = int.Parse(Console.ReadLine());

        if (value == RequiredSum(value))
        {
            Console.WriteLine("Armstrong Number");
        }
        else 
        {
            Console.WriteLine("Not an Armstrong Number");
        }
    }

    private static int CountDigits(int num)
    {
        int i = 0; 
        for (;num > 0; ++i) num /= 10;

        return i;    
    }

    private static int RequiredSum(int num)
    {
        int count = CountDigits(num);

        int sum = 0;
        while (num > 0)
        {
            sum += (int)Math.Pow(num % 10, count);
            num /= 10;
        }

        return sum;
    }
}

C

The following C program determines whether the integer entered is a Narcissistic / Armstrong number or not.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

int getNumberOfDigits(int n);
bool isArmstrongNumber(int candidate);

int main()
{
    int userNumber = 0;
    printf("Enter a number to verify if it is an Armstrong number: ");
    scanf("%d", &userNumber);
    printf("Is %d an Armstrong number?: %s\n", userNumber,  isArmstrongNumber(userNumber) ? "true" : "false");
    return 0;
}

bool isArmstrongNumber(int candidate)
{
    int numberOfDigits = getNumberOfDigits(candidate);
    int sum = 0;
    for (int i = candidate; i != 0; i /= 10)
    {
	    int num = i % 10;
	    int n = 1;
	    for (int j = 0; j < numberOfDigits; j++)
		{
			n *= num;
		}
	    sum += n;
    }
    return sum == candidate;
}

int getNumberOfDigits(int n)
{
    int sum = 0;
    while (n != 0)
    {
        n /= 10;
        ++sum;
    }
    return sum;
}

C++

The following C++ program determines whether the Integer entered is a Narcissistic / Armstrong number or not.

#include <iostream>
#include <cmath>
bool isArmstrong(int n)
{
    //The floor function is redundant because log10(n) + 1 will always be an integer when n is positive. Simply using static_cast<int>(log10(n)) + 1 would suffice.
    //int digits = floor(log10(n) + 1); //math formula to find number of digits in a number with any base
    int sum = 0;
    if (n >= 0)
    {
        int digits = static_cast<int>(log10(n)) + 1;
        for (int tmp = n; tmp; tmp /= 10) sum += pow(tmp%10, digits);
    }
    return sum == n;
}
int main()
{
    int n = 407;
    if(isArmstrong(n)) std::cout << n << " is a narcissistic number\n";
    else std::cout << n << " is not a narcissistic number\n";
}

Ruby

The following Ruby program determines whether the integer entered is a Narcissistic / Armstrong number or not.

def narcissistic?(value) #1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153
  nvalue = []
  nnum = value.to_s
  nnum.each_char do |num|
    nvalue << num.to_i
    end
  sum = 0
  i = 0
  while sum <= value
    nsum = 0
    nvalue.each_with_index do |num,idx|
      nsum += num ** i
    end
    if nsum == value
      return true
    else
      i += 1
      sum += nsum
    end
  end
return false
end

JavaScript

The following JavaScript program determines whether the integer entered is a Narcissistic / Armstrong number or not.

function narcissistic(number) {
  const numString = number.toString();
    const numDigits = numString.length;
    let sum = 0;

    for (let digit of numString) {
      sum += Math.pow(parseInt(digit), numDigits);
    }

    return sum === number;
}

Rust

The following Rust program prints all the Narcissistic / Armstrong numbers from 0 to 100 million in base 10.

fn is_armstrong_number(num: u64) -> bool {
    let digits = num.to_string();
    digits
        .chars()
        .map(|x| (x as u64 - 0x30).pow(digits.len() as u32))
        .sum::<u64>()
        == num
}

fn main() {
    (0..100_000_000).for_each(|n| {
        if is_armstrong_number(n) {
            println!("{n}")
        }
    })
}

References

^ ^a ^b ^c Weisstein, Eric W. "Narcissistic Number". MathWorld.
^ Perfect and PluPerfect Digital Invariants Archived 2007-10-10 at the Wayback Machine by Scott Moore
^ PPDI (Armstrong) Numbers by Harvey Heinz
^ Armstrong Numbers by Dik T. Winter
^ Lionel Deimel’s Web Log
^ (sequence A005188 in the OEIS)

Joseph S. Madachy, Mathematics on Vacation, Thomas Nelson & Sons Ltd. 1966, pages 163-175.
Rose, Colin (2005), Radical narcissistic numbers, Journal of Recreational Mathematics, 33(4), 2004–2005, pages 250-254.
Perfect Digital Invariants by Walter Schneider

External links

Digital Invariants
Armstrong Numbers
Armstrong Numbers in base 2 to 16
Armstrong numbers between 1-999 calculator
Symonds, Ria. "153 and Narcissistic Numbers". Numberphile. Brady Haran. Archived from the original on 2021-12-19.

[mw-1] Weisstein, Eric W. "Narcissistic Number". MathWorld.

[moore-2] Perfect and PluPerfect Digital Invariants Archived 2007-10-10 at the Wayback Machine by Scott Moore

[3] PPDI (Armstrong) Numbers by Harvey Heinz

[4] Armstrong Numbers by Dik T. Winter

[5] Lionel Deimel’s Web Log

[6] (sequence A005188 in the OEIS)

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