[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) › FibonacciIndex

```ts
const FibonacciIndex: brand(
  "FibonacciIndex",
  lessThanOrEqualTo(78)(PositiveInt),
);
```

Defined in: [packages/common/src/Number.ts:301](https://github.com/evoluhq/evolu/blob/dd96d79f1dbe9a49fa12ce8e0aa7d3d0177795ca/packages/common/src/Number.ts#L301)

Valid index for [fibonacciAt](https://evolu.dev/docs/api-reference/common/Number/functions/fibonacciAt), constrained to 1-78.

Limited to 78 because F(79) exceeds JavaScript's `MAX_SAFE_INTEGER`.