OFFSET
1,1
COMMENTS
Nontrivial means that there must be at least two factors.
For any a(n), 10*a(n) is also in the sequence, and also in A144563. - M. F. Hasler, Nov 01 2021
REFERENCES
Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 65.
LINKS
Ely Golden, Table of n, a(n) for n = 1..1000 (corrected by Sean A. Irvine)
Gordon Hamilton, Three integer sequences from recreational mathematics, Video (2013).
Sean A. Irvine, Java program (github)
EXAMPLE
1395 = 31*9*5, so 1395 is a term.
179739 = 7 * 9 * 9 * 317 so 179739 is a term. - Sean A. Irvine, Feb 28 2023
PROG
(PARI) is_A020342(n, m=0, D=vecsort(digits(n)))={ if(m && n >= m && vecsort(digits(n))==D, 1, #D<3, m && (D[1]>=m && vecprod(D)==n), n >= m^2, my(S=Set(D), i, C); fordiv(n, f, f < m && next; f*f > n && break; setminus(Set(digits(f)), S) && next; C=D; foreach(digits(f), d, if(i=vecsearch(C, d), C=C[^i], next(2))); C && is_A020342(n\f, f, C) && return(1)))} \\ See A144563 for a function counting the multiplicity of the representation. - M. F. Hasler, Nov 01 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
EXTENSIONS
Edited by N. J. A. Sloane, Jan 03 2009
STATUS
approved