# is a number contained within this number series

I hope I can ask this question with enough clarity. It is not exactly clear in my head., but here goes

I have a number series, it starts at a random number and then increments by 8(always by 8) plus the last number.

for example in the series:

36,80,132,192,260,336,420,512,612,720,836,960


We start at 36 (randomly chosen)

We then add 8 to 36 to make 44

We then add 44 to 36 to make the next number in the series 80

Then we add 8 to 44 to make 52 and add that to 80 to make the next number in the series 132

So we have two sets of numbers one which is incrementing by 8 the other which is the addition of the incrementing numbers.

 36, 44, 52, 60, 68, 76, 84, 92,100,108,116,124
36, 80,132,192,260,336,420,512,612,720,836,960 <== target series


Now I want to test whether another number exists in that series for example we can see that 612 is valid for that series but 680 is not.

Is the number 6992 a valid number for that series?

Is there a way I can test for validity without having to calculate every number in the series up until the one I want?

Is there a way I can count how many iterations I'd need to go through to get to the number. For example 612 is the 8th iteration.

This is part of a programming task (not school work but special interest) I will have tens of millions of these to calculate and I want the program to run as fast and as efficiently as possible.

So any help will be appreciated. If it can't be done then so be it, but not being a mathematician I thought I better ask before I gave up.

DC

• Your sequence is determined by the recurrence relation $a_{n+1} = a_n + a_0 + 8(n+1)$. Nov 15 '13 at 13:11

As mentioned by Antonio, the recurrence relation that defines this sequence is: $$a_n = a_{n-1} + a_0 + nb \,,$$ where in the example above $a_0 = 36$ and $b=8$. A little thought will show that, generally, $$a_n = (n+1)a_0 + b\sum_{k=1}^n k = (n+1)a_0 + \frac{n(n+1)}{2}b \,.$$ This can be proved simply by induction but was deduced by considering the following: $$a_0 = a_0 \\ a_1 = a_0 + a_0 + b = 2a_0 + b \\ a_2 = a_1 + a_0 + 2b = (2a_0 + b) + a_0 + 2b = 3a_0 + (1+2)b \\ a_3 = a_2 + a_0 + 3b = (3a_0 + (1+2)b) + a_0 + 3b = 4a_0 + (1+2+3)b \\ \ldots \\ a_n = (n+1)a_0 + b\sum_{k=1}^n k$$ Now that we have a formula to compute the $n$th number in the sequence without computing the previous numbers we can formulate a way to test any number to see if it is in the sequence by solving this for $n$, which results in the following quadratic equation: $$\frac{b}{2}n^2 + \left(\frac{b}{2} + a_0\right)n + (a_0 - a_n) = 0\,,$$ where $a_n$ is the number you wish to test. If you solve for $n$ using the quadratic formula and one of the solutions is a non-negative integer then $a_n$ is in the sequence. Be careful implementing this to see if this is an integer because floating point arithmetic can be imprecise.