f(x) = 3x + 1 (mod p).
Since gcd(3, p) = 1, f is an invertible affine transformation. In particular, f is a permutation of ℤ/pℤ. Moreover, if we write f in the form
f(x) = 3(x + c)
with the unique constant c = 1/3 (interpreted in ℤ/pℤ) (i.e. by solving 3x + 1 = 3(x + c)), it follows that f is conjugate to the multiplication-by-3 map on ℤ/pℤ and hence its cycle structure is completely determined by the order of 3 modulo p.
In particular, for any starting value α ∈ ℤ/pℤ not equal to the unique fixed point, the orbit {α, f(α), f²(α), …} is a (cyclic) subgroup of ℤ/pℤ and hence "exhausts" its cycle. 3n + 1 ≡ 1 (mod 3).
Thus, if one defines L(n)=3n+1 (the “linear part” of the Collatz map), then for every n ∈ ℤ we have L(n) ∈ {1} (mod 3); in other words, the mapping L “eliminates” the residue classes 0 and 2 modulo 3.
Proof. A/2^k
to be the unique element of ℤ/pℤ equal to A · (2^k)⁻¹, where (2^k)⁻¹ is the multiplicative inverse of 2^k modulo p. Then the operation “division by 2^k” is an automorphism of ℤ/pℤ. In particular, if A ≡ c (mod p) then
A/2^k ≡ c · (2^k)⁻¹ (mod p),
so any congruence property of A modulo p is preserved (up to multiplication by the invertible constant (2^k)⁻¹).
Proof. R = ∏{p∈S} R_p
which is a proper subset of
X = ∏{p∈S} ℤ/pℤ.
In particular, by the Chinese remainder theorem the total number of residue combinations the future value of n may assume is at most ∏{p∈S} |R_p|, strictly less than ∏{p∈S} p. Thus, under successive iterations the set of “admissible” residue vectors (that is, the overall modular configuration of n) is contracted.
Proof. f(x) = 3x + 1 (mod p).
Since gcd(3, p) = 1, f is an invertible affine transformation. In particular, f is a permutation of ℤ/pℤ. Moreover, if we write f in the form f(x) = 3(x + c)
with the unique constant c = 1/3 (interpreted in ℤ/pℤ) (i.e. by solving 3x + 1 = 3(x + c)), it follows that f is conjugate to the multiplication-by-3 map on ℤ/pℤ and hence its cycle structure is completely determined by the order of 3 modulo p. 3n + 1 ≡ 1 (mod 3).
Thus, if one defines L(n)=3n+1 (the “linear part” of the Collatz map), then for every n ∈ ℤ we have L(n) ∈ {1} (mod 3); in other words, the mapping L “eliminates” the residue classes 0 and 2 modulo 3. A/2^k
to be the unique element of ℤ/pℤ equal to A · (2^k)⁻¹, where (2^k)⁻¹ is the multiplicative inverse of 2^k modulo p. Then the operation “division by 2^k” is an automorphism of ℤ/pℤ. In particular, if A ≡ c (mod p) then A/2^k ≡ c · (2^k)⁻¹ (mod p),
so any congruence property of A modulo p is preserved (up to multiplication by the invertible constant (2^k)⁻¹). R = ∏{p∈S} R_p
which is a proper subset of X = ∏{p∈S} ℤ/pℤ.
In particular, by the Chinese remainder theorem the total number of residue combinations the future value of n may assume is at most ∏{p∈S} |R_p|, strictly less than ∏{p∈S} p. Thus, under successive iterations the set of “admissible” residue vectors (that is, the overall modular configuration of n) is contracted.
[edit] I have deleted the half-ass ChatGPT generated Lean because it doesn't compile yet and that's lame. Working on it.