Problem Statement

The objective is to write a function that finds the sum of all positive multiples of 3 or 5 below n. Show output for n = 1000. This is is the same as Project Euler problem 1. Extra credit: do this efficiently for n = 1e20 or higher.

Let's start with the solution:

Sum ← +´·(0=3⊸|⌊5⊸|)⊸/↕


Sum ← {
  m ← (0=3⊸|⌊5⊸|)↕15 ⋄ h‿l ← 15(⌊∘÷˜∾|)𝕩
  (+´l↑m×15(×+↕∘⊣)h) + (15×(+´m)×2÷˜h×h-1) + h×+´m×↕15
}