There are "bignum" implementations for every language. Though I never tested the performance impact on closed-form Fibonacci when a defined double precision >64

There are "bignum" implementations for every language. Though I never tested the performance impact on closed-form Fibonacci when a defined double precision >64bit is used. https://gmplib.org/ https://gmplib.org/ Your code has an accidentally quadratic runtime (instead of linear). Since the array is appended to, the code regularly increases the memory region and has to move all the previous data over. You could pre-allocate the memory as n is known ahead. Also you don't need all that memory. You only need to store the last two entries.