Repdigit Niven-number parametrization conjecture

Let a(k)a_{(k)} denote the kk-digit repdigit formed by repeating the digit aa, and let a{1,2,3,4,5,6,7,8,9}a\in\{1,2,3,4,5,6,7,8,9\}. A repdigit is a Niven number when

10k1(mod9ka).10^k\equiv 1\pmod{9ka}.

Define

m0=37,m1=333667,m21=757,m22=440334654777631,m_0=37,\quad m_1=333667,\quad m_{21}=757,\quad m_{22}=440334654777631, m31=163,m32=9937,m33=2462401,m34=676421558270641,m_{31}=163,\quad m_{32}=9937,\quad m_{33}=2462401,\quad m_{34}=676421558270641, m35=130654897808007778425046117.m_{35}=130654897808007778425046117.

Repdigit Niven-number parametrization conjecture. The solutions of the repdigit Niven-number equation are parameterized by

k=k{nαβγδ}=3nm0αm1βm21γ1m22γ2m31δ1m32δ2m33δ3m34δ4m35δ5,k=k_{\{n\alpha\beta\gamma\delta\}}=3^n\cdot m_0^\alpha\cdot m_1^\beta\cdot m_{21}^{\gamma_1}\cdot m_{22}^{\gamma_2}\cdot m_{31}^{\delta_1}\cdot m_{32}^{\delta_2}\cdot m_{33}^{\delta_3}\cdot m_{34}^{\delta_4}\cdot m_{35}^{\delta_5},

where

{α,β,γ1,γ2,δ1,δ2,,δ5}N0\{\alpha,\beta,\gamma_1,\gamma_2,\delta_1,\delta_2,\ldots,\delta_5\}\in\mathbb{N}_0

and the minimal-index constraints are

n0if all exponents are zero,n\geq 0\quad\text{if all exponents are zero}, n1if α0,n2if β0,n\geq 1\quad\text{if }\alpha\neq 0,\qquad n\geq 2\quad\text{if }\beta\neq 0, n3if {γ1,γ2}{0},n4if {δ1,,δ5}{0}.n\geq 3\quad\text{if }\{\gamma_1,\gamma_2\}\neq\{0\},\qquad n\geq 4\quad\text{if }\{\delta_1,\ldots,\delta_5\}\neq\{0\}.

The parametrization is intended to demonstrate infinitely many repdigit Niven numbers, which are consequently permutation-invariant Niven numbers; the source does not provide evidence resolving whether this proposed parametrization is complete.

Sources & referencesView supporting material

Primary source

Hui-Ling Wu and S. Y. Lou, “Permutation–invariant Niven numbers”, arXiv:2508.01611 (2025).

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