I made some research and I believe that the numbers are results of a polynomial (ring) function.
It also seems that someone already got into this in February 2015. Probably the guy who cashed out the first addresses.
http://pastebin.com/erN0F1ce-673909/1307674368000*x^15 + 5004253/87178291200*x^14 - 151337/52254720*x^13 + 9320029/106444800*x^12 - 25409989753/14370048000*x^11 + 2192506957/87091200*x^10 - 19011117413/73156608*x^9 + 1200887962891/609638400*x^8 - 3585932821063/326592000*x^7 + 647416874047/14515200*x^6 - 18586394742863/143700480*x^5 + 30899291755337/119750400*x^4 - 274137631043849/825552000*x^3 + 36933161067083/151351200*x^2 - 87781079/1155*x
If you put for x the values 0 to 15, you will get exactly the first 16 numbers.
1
3
7
8
21
49
76
224
467
514
1155
2683
5216
10544
26867
51510
I just checked this more closely, looks like we are back to step 0.
That pastebin entry are inputs/outputs of an application called sage, I actually downloaded it and was playing around with it.
For those interested:
http://www.sagemath.org/Problem is, that this just proves that there is no mathematical formula behind the sequence.
You can see in the first part, he placed already the sequence in the inputs:
R=PolynomialRing(QQ,'x')
sage: f = R.lagrange_polynomial([(0,
1),(1,
3),(2,
7),(3,
8),(4,
21),(5,
49),(6,
76),(7,
224),(8,
467),(9,
514),(10,
1155),(11,
2683),(12,
5216),(13,
10544),(14,
26867),(15,
51510)]);
Those values were placed in the variable f.
Then he asked for the output of f, and sage returned the formula based on those actual fixed values:
[b]f[/b]
-673909/1307674368000*x^15 + 5004253/87178291200*x^14 - 151337/52254720*x^13 + 9320029/106444800*x^12 - 25409989753/14370048000*x^11 + 2192506957/87091200*x^10 - 19011117413/73156608*x^9 + 1200887962891/609638400*x^8 - 3585932821063/326592000*x^7 + 647416874047/14515200*x^6 - 18586394742863/143700480*x^5 + 30899291755337/119750400*x^4 - 274137631043849/825552000*x^3 + 36933161067083/151351200*x^2 - 87781079/1155*x + 1
When this formula is applied for the rest of the sequence that is not part of the inputs, it spits out nothing of value.
Apparently it just proves that there is no formula behind this sequence, at least using the Polynomial Ring model.
This was probably a test done by someone in the past that already realized this transaction in the blockchain, but the test failed.
So basically we are back to the one and only brute force impossible option.