[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Number](https://evolu.dev/docs/api-reference/common/Number) › fibonacciAt

```ts
function fibonacciAt(
  index: number &
    Brand<"Int"> &
    Brand<"NonNegative"> &
    Brand<"Positive"> &
    Brand<"LessThanOrEqualTo78"> &
    Brand<"FibonacciIndex">,
): number & Brand<"Int"> & Brand<"NonNegative"> & Brand<"Positive">;
```

Defined in: [packages/common/src/Number.ts:113](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Number.ts#L113)

Returns the Fibonacci number at the given index (1-indexed: 1,1,2,3,5,8,...).