[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
type FibonacciIndex = number &
  import("./Brand.ts").Brand<"NonNaN"> &
  import("./Brand.ts").Brand<"Finite"> &
  import("./Brand.ts").Brand<"Int"> &
  import("./Brand.ts").Brand<"NonNegative"> &
  import("./Brand.ts").Brand<"Positive"> &
  import("./Brand.ts").Brand<"LessThanOrEqualTo78"> &
  import("./Brand.ts").Brand<"FibonacciIndex">;
```

Defined in: [packages/common/src/Number.ts:233](https://github.com/evoluhq/evolu/blob/037c390af081e9944d616ff298a729515c5c5ab7/packages/common/src/Number.ts#L233)

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`.