[svat]

Ah, clicking the footnote numbers worked for footnotes 1 to 4, but not for 5 and 6.

BTW, for the question “Where do the constant values 0, 6, 10, and 1 come from?”, though it's implicit in the post, it is useful to note explicitly that as x^4 takes only the values 0 or 1 either mod 3 or mod 5, these four possibilities—namely (0,0), (0,1), (1,0), (1,1)—precisely account for those values:

    (0 mod 3) and (0 mod 5) ⇔ (0 mod 15)
    (0 mod 3) and (1 mod 5) ⇔ (6 mod 15)
    (1 mod 3) and (0 mod 5) ⇔ (10 mod 15)
    (1 mod 3) and (1 mod 5) ⇔ (1 mod 15)

This would also simplify the post considerably maybe, as much of the algebra wouldn't be needed.