[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<"NonNaN"> &
    Brand<"Finite"> &
    Brand<"Int"> &
    Brand<"NonNegative"> &
    Brand<"Positive"> &
    Brand<"LessThanOrEqualTo78"> &
    Brand<"FibonacciIndex">,
): number &
  Brand<"NonNaN"> &
  Brand<"Finite"> &
  Brand<"Int"> &
  Brand<"NonNegative"> &
  Brand<"Positive">;
```

Defined in: [packages/common/src/Number.ts:240](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Number.ts#L240)

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