USACO Arithmetic Progressions
Welcome to CodeReview!

Whitespace

The PEP8 standard, and consequently most Python linting tools, will recommend that you add another linebreak before your function definitions, plus some whitespace around your operators, etc. I won't detail this exhaustively; you're best to use the IDE of your choice - PyCharm is a reasonable one that is helpful for this.

Type hinting

bound is probably an integer, so add : int after it. It probably returns an int as well.

Subroutines

Put your global-scoped code into subroutines for ease of maintenance, legibility and testing.

Redundant pass

That pass isn't needed.

Use format strings

This:

str(i[0])+' '+str(i[1])+'\n'


can be

f'{i[0]} {i[1]}\n'


Simplify some math

This:

((bound**2)*2)+1


can be

2 * bound**2 + 1


due to operator precedence.

Truthiness

This:

if parity[a+n*d] != 1:


can be

if not parity[a + n*d]:


because 0 is falsey.

camel_case

ariLen is more commonly written ari_len in Python.