Innumber theory, anarcissistic number^[1][2] (also known as apluperfect digital invariant (PPDI),^[3] anArmstrong number^[4] (after Michael F. Armstrong)^[5] or aplus perfect number)^[6] in a givennumber base${\displaystyle b}$ is a number that is the sum of its own digits each raised to the power of the number of digits.

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

${\displaystyle F_b(n)=\sum _{i=0}^{k-1}{d}_i^k.}$

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

${\displaystyle d_i=\frac{nmod{b}^{i+1}-n{modb}^i}{b^i}}$

is the value of each digit of the number. A natural number${\displaystyle n}$ is anarcissistic number if it is afixed point for${\displaystyle F_b}$, which occurs if${\displaystyle F_b(n)=n}$. The natural numbers aretrivial narcissistic numbers for all${\displaystyle b}$, all other narcissistic numbers arenontrivial narcissistic numbers.

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

A natural number${\displaystyle n}$ is asociable narcissistic number if it is aperiodic point for${\displaystyle F_b}$, where${\displaystyle F_b^p(n)=n}$ for a positiveinteger${\displaystyle p}$ (here${\displaystyle F_b^p}$ is the${\displaystyle p}$thiterate of${\displaystyle F_b}$), and forms acycle of period${\displaystyle p}$. A narcissistic number is a sociable narcissistic number with${\displaystyle p=1}$, and anamicable narcissistic number is a sociable narcissistic number with${\displaystyle p=2}$.

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

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

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

Every base${\displaystyle b\ge 3}$ 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, ... (sequenceA248970 in theOEIS)

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]

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

${\displaystyle b}$	Narcissistic numbers	#	Cycles	OEIS sequence(s)
2	0, 1	2	${\displaystyle \varnothing }$
3	0, 1, 2, 12, 22, 122	6	${\displaystyle \varnothing }$
4	0, 1, 2, 3, 130, 131, 203, 223, 313, 332, 1103, 3303	12	${\displaystyle \varnothing }$	A010344 andA010343
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 andA010351
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

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

• Arithmetic dynamics
• Dudeney number
• Factorion
• Happy number
• Kaprekar's constant
• Kaprekar number
• Meertens number
• Perfect digit-to-digit invariant
• Perfect digital invariant
• Sum-product number

• Digital Invariants
• Armstrong Numbers
• Armstrong Numbers in base 2 to 16
• Armstrong numbers between 1-999 calculator
• Symonds, Ria (3 January 2012)."153 and Narcissistic Numbers".Numberphile.Brady Haran.Archived from the original on 2021-12-19.

• Arithmetic dynamics
• Base-dependent integer sequences

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