OFFSET
1,1
COMMENTS
Numbers n such that numbers of numbers that occur in the concatenation of 1,2,3...,n-1 equals numbers of numbers that do not occur in the concatenation of 1,2,3...,n-1.
There are no other terms <= 600000. The plots in the link strongly suggest that the sequence is complete.
LINKS
Klaus Brockhaus, Plots of A131981(n)/n at various scales
EXAMPLE
A131981(n) < n/2 for 1 <=n < 576,
A131981(n) < n/2 for 576 < n < 584,
A131981(n) > n/2 for 584 < n < 588,
A131981(n) < n/2 for 588 < n < 592,
A131981(n) > n/2 for 592 < n < 600,
A131981(n) > n/2 for 600 < n < 1650,
A131981(n) > n/2 for 1650 < n < 1654,
A131981(n) < n/2 for 1654 < n < 3430,
A131981(n) > n/2 for 3430 < n < 3440,
..............
A131981(n) < n/2 for 3608 < n <= 3610,
A131981(n) > n/2 for 3610 < n <= 600000.
PROG
(JBASIC)
s$ = "" : c = 0 : d = 0
FOR n = 1 TO 4000
sn$ = str$(n)
IF instr(s$, sn$) > 0 THEN d = d+1 ELSE c = c+1
s$ = s$ + sn$ : IF c = d THEN print n ; ", ";
NEXT
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Klaus Brockhaus, Aug 15 2007
EXTENSIONS
Edited by Charles R Greathouse IV, Oct 28 2009
STATUS
approved