OFFSET
1,1
COMMENTS
Numbers that are the sum from their more significant digits counted down to the following digits. The first is the 19th triangular number: 190 = 19 + 18 + 17 + ... + 1 + 0.
See A186074 for numbers that sum by counting upward.
An alternate definition: n = sum from x to y inclusive = A000217(y) - A000217(x-1), (A000217 are the triangular numbers) where the digits of n are the concatenation of y and x.
These are the positive integer solutions to the equation Sum_{i=x..y} i = (10^k)*y + x, where 0 <= x < y, 0 <= x < 10^k, k = 1,2,3...
The graph of the function is a hyperbola; the solutions are for positive x and y, where x does not "overlap" and add to y. The first 15 terms are all of the solutions for m = 1 to 3.
Note that terms A186074(4) and A186074(10) have trailing 0's, i.e. 19900 = Sum_{k=0..199} k and 1999000 = Sum_{k=0..1999} k. Strictly speaking, these do not meet the concatenation criterion. This pattern continues indefinitely: 199990000, 19999900000, etc. - Matthew Goers, Jun 03 2011
All terms form (10^k)*y + x, where y = (s+t-1)/2 + 10^k, x = (s-t-1)/2, s*t = 100^k - 10^k, 0 <= (s-t-1)/2 < 10^k, and gcd(s, t) is an odd number. - Jinyuan Wang, Sep 13 2019
LINKS
Jinyuan Wang, Table of n, a(n) for n = 1..21167
EXAMPLE
204 = 20 + 19 + 18 + ... + 5 + 4.
2002077 = 2002 + 2001 + ... + 78 + 77.
2006118 = 2006 + 2005 + ... + 119 + 118.
PROG
(PARI) do(s, t, k) = if(s - t > 0 && (s-t-1)/2 < 10^k, (10^k-1+s)*(10^k+1+t)/2, 204);
lista(nn) = {my(v=List()); for(k = 1, nn, fordiv(50^k - 5^k, s, t = (100^k-10^k)/s; listput(v, do(s, t, k)); listput(v, do(2^k*s, t/2^k, k)))); Set(v); } \\ Jinyuan Wang, Sep 13 2019
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Matthew Goers, Feb 11 2011
EXTENSIONS
Missing term a(4) = 19900 inserted by Matthew Goers, Jun 03 2011
a(16)-a(28) from Donovan Johnson, Aug 22 2012
STATUS
approved