What is the pattern to this sequence?

Question

$$0, 1, 3, 13, 51, 205$$

More specifically, $$(0,0)\quad(1,1)\quad (2,3)\quad (3,13) \quad(4,51)\quad (5,205)$$ I have tried using the interpolation feature in Grapher.app and Wolfram Alpha, but cannot seem to get what I'm looking for which is a quadratic function that hands me the numbers from the top part.

UPDATE/EDIT

For those of you interested, I encountered the above sequence after factoring $\frac{2}{5}$ from the sum $$\sum_{n=1}^\infty \frac{2^n}{2^{2n} + (-1)^{n+1}} = \frac{2}{5} +\frac{4}{15}+\frac{8}{65}+\frac{16}{255}+\frac{32}{1025} ...$$

$$=\frac{2}{5} * \left(1+ \frac{2}{3}+\frac{4}{13}+\frac{8}{51}+\frac{16}{205} ...\right)$$

The numerators in the brackets are powers of $2$, while the denominators were the ones in question.

Thanks for the help!

Answer 1

In cases like this, it is recommended to try and visit the OEIS.

Answer 2

Of course, with such pattern recognition questions, you can let the next value be anything.

My suspicion is that $u_{n+1} = 4 u_{n} + (-1)^n$.

Answer 3

Using the recurrence $a_n = 4a_{n-2} +3a_{n-1}$, which this sequence appears to satisfy, you can derive an explicit formula for $a_n$, namely:

$a_n = \frac{4^n - (-1)^n}{5}$

It's exponential - it grows too fast to be quadratic. You can find a quadratic function that passes through any three of those points, but it won't pass through the others at the same time.

Answer 4

For a question like this we can come up with our own pattern. The next number can be anything we want. We can come up with a pattern for each answer.

Answer 5

There is no quadratic function for that sequence. One possible equation is: $ a(n) = 3a(n-1) + 4a(n-2)$

This sequence represents:

• The inverse binomial transform of powers of 5.
• The number of walks of length n between any two distinct vertices of the complete graph $K_5$.
• The number of segments (sides) per iteration of the space-filling Peano-Hilbert curve.
• For n>=2, a(n) equals the permanent of the (n-1)X(n-1) tridiagonal matrix with 3's along the central diagonal, and 2's along the subdiagonal and the superdiagonal.

Source: the online encyclopedia of integer sequences.

Answer 6

a(n)=a(n-1)*4 then check the value of n position, if it is odd then add one,if it is even then substrata one