OFFSET
1,1
COMMENTS
The first even element of this sequence is a(817) = 3464.
EXAMPLE
Let T(i,j) be the triangle read by rows: T(i,j) = 1 if i mod j = floor((i - j)/j) mod j, T(i,j) = 0 otherwise, for 1 <= j <= i. The triangle begins:
i\j| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ...
-----------------------------------------
1| 1
2| 1 1
3| 1 0 1
4| 1 0 0 1
5| 1 1 0 0 1
6| 1 1 0 0 0 1
7| 1 0 1 0 0 0 1
8| 1 0 0 0 0 0 0 1
9| 1 1 0 1 0 0 0 0 1
10| 1 1 0 0 0 0 0 0 0 1
11| 1 0 1 0 1 0 0 0 0 0 1
12| 1 0 1 0 0 0 0 0 0 0 0 1
13| 1 1 0 0 0 1 0 0 0 0 0 0 1
14| 1 1 0 1 0 0 0 0 0 0 0 0 0 1
15| 1 0 0 0 0 0 1 0 0 0 0 0 0 0 1
...
The j-th column has period j^2. Consecutive elements of this period are j X j identity matrix entries, read by rows.
11 is not in this sequence because only k's <= 11 satisfying the equation 11 mod k = floor((11 - k)/k) mod k are: 1, 3, 5, 11, hence 1+3+5+11 = 20 and 20 < 2*11.
23 is in this sequence because only k's <= 23 satisfying the equation 23 mod k = floor((23 - k)/k) mod k are: 1, 5, 7, 11, 23, hence 1+5+7+11+23 = 47 and 47 > 2*23.
PROG
(Maxima)
(f(i, j):=mod(i-floor((i-j)/j), j),
(n:0, for m:2 thru 500 do
(s:0, for k:1 thru floor(m/2) do
(if f(m, k)=0 then
(s:s+k)), if s>m then
(n:n+1, print(n , "" , m)))));
CROSSREFS
KEYWORD
nonn
AUTHOR
Lechoslaw Ratajczak, Aug 20 2024
STATUS
approved