OFFSET
0,1
COMMENTS
a(n) is the smallest number k such that A007954(A000203(k)) = n in base 10, or 0 if no such number exists.
If we write -1 to indicate that no solution has so far been found for n, then the present state of the sequence is as follows: 19, 1, 6, 2, 3, 8, 5, 4, 7, 36, 111, 0, 61, 0, 30, 5041, 12, 0, 22, 0, 34, -1, 0, 0, 37, -1, 0, 18, 73, 0, 28, 0, 33, 0, 0, 49, 193, 0, 0, 0, 157, 0, 129, 0, 0, 72, 0, 0, 60, -1, 128, 0, 0, 0, 42, 0, 45, 0, 0, 0, 217, 0, 0, 12800, 112, 0, 0, 0, 0, 0, 387, 0.
No terms found below 10^9. - Michel Marcus, Apr 08 2015
If k exists for n = 21, 25, 49, 125 or 245, it must be bigger than 10^20; if k exists for n = 343, 375, 525, 625, 675, 729 or 735, it must be bigger than 10^18. - Jinyuan Wang, Nov 01 2020
If they exist, terms a(21), a(25), a(49) are greater than 10^59. - Max Alekseyev, Feb 20 2024
FORMULA
If p = prime > 7 and m >= 1, then a(mp) = 0.
PROG
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Jaroslav Krizek, Apr 06 2015
EXTENSIONS
Edited by N. J. A. Sloane, Apr 06 2015
STATUS
approved