# Finding an unknown number in the middle of a number sequence

I've stumbled across a very peculiar number sequence and it was bugging me for a while now, i would really like to find out a way to dissect and work out the number that is missing, here's the sequence:

$1010, 1001, 111, 100, ?, 111, 10, 110$

I've tried multitude of algorithms under decimal, hex, octal or binary representations of this sequence but to no avail. I'm starting to wonder if this is a trick question or not.

Help of any form is greatly appreciated.

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Context, please. Where did you see this sequence? – M. Vinay Jun 17 '14 at 15:18
That was a first question in an interview my friend did online (developer job if that gives it some sorts of context). If you are curious here is a straight copy of the question - "Find the missing number in the series". That is it.. no other info unfortunately. – kosdos Jun 18 '14 at 8:04
There is a pattern up to the missing number, which seems to be broken after it: $1010 - 1 = 1001, 1001 - 10 = 111, 111 - 11 = 100, 100 - 100 = 0$, so $0$ should be the missing number (according to this). But after that it should be $0 - 101 = -101$, or it could be $2$'s complement subtraction, but that doesn't work either (that would be $1011$, assuming four bits). – M. Vinay Jun 18 '14 at 8:09
That's interesting, i might need to look more into this, thanks for pointing out 2's complement subtraction. Personally i was thinking this is somewhat related to an alternating permutations since the numbers go up and down, or something similar to alternating permutation which i'm not aware of.. – kosdos Jun 18 '14 at 8:19