# 1. [Triangular], 10 bytes, [A000217]

 $\:_%i/2*<

[Try it online!][TIO-j5dzkkcr]

[Next Sequence]

## How it works

The code formats into this triangle

 $
 \ :
 _ % i
 / 2 * <

with the IP starting at the `$` and moving South East (SE), works like this:

 $ Take a numerical input (n); STACK = [n]
 : Duplicate it; STACK = [n, n]
 i Increment the ToS; STACK = [n, n+1]
 < Set IP to W; STACK = [n, n+1]
 * Multiply ToS and 2ndTos; STACK = [n(n+1)]
 2 Push 2; STACK = [n(n+1), 2]
 / Set IP to NE; STACK = [n(n+1), 2]
 _ Divide ToS by 2ndToS; STACK = [n(n+1)/2]
 \ Set IP to SE; STACK = [n(n+1)/2]
 % Output ToS as number; STACK = [n(n+1)/2]
 * Multiply ToS by 2ndToS (no op); STACK = [n(n+1)/2]

[Triangular]: https://github.com/aaronryank/triangular
[TIO-j5dzkkcr]: https://tio.run/##KynKTMxLL81JLPr/XyXGKl41U99Iy@b/f0MDAA "Triangular – Try It Online"

[A000217]: https://oeis.org/A000217
[Next Sequence]: https://oeis.org/A000010