METHOD FOR OBTAINING
https://oeis.org/A273315 : “First differences (i.e., the differences between successive terms) of the number of active (lit, black) cells in the nth growth stage of a two-dimensional cellular automaton defined by the ‘643 Rule’, based on the 5-cell von Neumann neighborhood.”
DATA
4, 12, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, 248, 256, 264, 272, 280, 288, 296, 304, 312, 320, 328, 336, 344, 352, 360, 368, 376, 384, 392, 400, 408, 416, 424, 432
METHOD
Consider the minimum k greater than a(k-1) such that (n-1)!+k is divisible by n.
EXAMPLES
n. k
1. 1
2. 4
3. 6
4. 11
5. 12
...
CONSIDERATION
Note that for odd n and starting from n=3, k*2 gives the above sequence.
n.b.
https://oeis.org/A316571