# mathlibdocumentation

algebra.archimedean

# Archimedean groups and fields. #

This file defines the archimedean property for ordered groups and proves several results connected to this notion. Being archimedean means that for all elements x and y>0 there exists a natural number n such that x ≤ n • y.

## Main definitions #

• archimedean is a typeclass for an ordered additive commutative monoid to have the archimedean property.
• archimedean.floor_ring defines a floor function on an archimedean linearly ordered ring making it into a floor_ring.
• round defines a function rounding to the nearest integer for a linearly ordered field which is also a floor ring.

## Main statements #

• ℕ, ℤ, and ℚ are archimedean.
@[class]
structure archimedean (α : Type u_2)  :
Prop
• arch : ∀ (x : α) {y : α}, 0 < y(∃ (n : ), x n y)

An ordered additive commutative monoid is called archimedean if for any two elements x, y such that 0 < y there exists a natural number n such that x ≤ n • y.

Instances
theorem linear_ordered_add_comm_group.exists_int_smul_near_of_pos {α : Type u_1} [archimedean α] {a : α} (ha : 0 < a) (g : α) :
∃ (k : ), k a g g < (k + 1) a

An archimedean decidable linearly ordered add_comm_group has a version of the floor: for a > 0, any g in the group lies between some two consecutive multiples of a.

theorem linear_ordered_add_comm_group.exists_int_smul_near_of_pos' {α : Type u_1} [archimedean α] {a : α} (ha : 0 < a) (g : α) :
∃ (k : ), 0 g - k a g - k a < a
theorem exists_nat_gt {α : Type u_1} [nontrivial α] [archimedean α] (x : α) :
∃ (n : ), x < n
theorem exists_nat_ge {α : Type u_1} [archimedean α] (x : α) :
∃ (n : ), x n
theorem add_one_pow_unbounded_of_pos {α : Type u_1} [nontrivial α] [archimedean α] (x : α) {y : α} (hy : 0 < y) :
∃ (n : ), x < (y + 1) ^ n
theorem pow_unbounded_of_one_lt {α : Type u_1} [archimedean α] (x : α) {y : α} (hy1 : 1 < y) :
∃ (n : ), x < y ^ n
theorem exists_nat_pow_near {α : Type u_1} [archimedean α] {x y : α} (hx : 1 x) (hy : 1 < y) :
∃ (n : ), y ^ n x x < y ^ (n + 1)

Every x greater than or equal to 1 is between two successive natural-number powers of every y greater than one.

theorem exists_int_gt {α : Type u_1} [archimedean α] (x : α) :
∃ (n : ), x < n
theorem exists_int_lt {α : Type u_1} [archimedean α] (x : α) :
∃ (n : ), n < x
theorem exists_floor {α : Type u_1} [archimedean α] (x : α) :
∃ (fl : ), ∀ (z : ), z fl z x
theorem exists_int_pow_near {α : Type u_1} [archimedean α] {x y : α} (hx : 0 < x) (hy : 1 < y) :
∃ (n : ), y ^ n x x < y ^ (n + 1)

Every positive x is between two successive integer powers of another y greater than one. This is the same as exists_int_pow_near', but with ≤ and < the other way around.

theorem exists_int_pow_near' {α : Type u_1} [archimedean α] {x y : α} (hx : 0 < x) (hy : 1 < y) :
∃ (n : ), y ^ n < x x y ^ (n + 1)

Every positive x is between two successive integer powers of another y greater than one. This is the same as exists_int_pow_near, but with ≤ and < the other way around.

theorem exists_pow_lt_of_lt_one {α : Type u_1} [archimedean α] {x y : α} (hx : 0 < x) (hy : y < 1) :
∃ (n : ), y ^ n < x

For any y < 1 and any positive x, there exists n : ℕ with y ^ n < x.

theorem exists_nat_pow_near_of_lt_one {α : Type u_1} [archimedean α] {x y : α} (xpos : 0 < x) (hx : x 1) (ypos : 0 < y) (hy : y < 1) :
∃ (n : ), y ^ (n + 1) < x x y ^ n

Given x and y between 0 and 1, x is between two successive powers of y. This is the same as exists_nat_pow_near, but for elements between 0 and 1

theorem sub_floor_div_mul_nonneg {α : Type u_1} [floor_ring α] (x : α) {y : α} (hy : 0 < y) :
0 x - (x / y) * y
theorem sub_floor_div_mul_lt {α : Type u_1} [floor_ring α] (x : α) {y : α} (hy : 0 < y) :
x - (x / y) * y < y
@[protected, instance]
@[protected, instance]
def archimedean.floor_ring (α : Type u_1) [archimedean α] :

A linear ordered archimedean ring is a floor ring. This is not an instance because in some cases we have a computable floor function.

Equations
theorem archimedean_iff_nat_lt {α : Type u_1}  :
∀ (x : α), ∃ (n : ), x < n
theorem archimedean_iff_nat_le {α : Type u_1}  :
∀ (x : α), ∃ (n : ), x n
theorem exists_rat_gt {α : Type u_1} [archimedean α] (x : α) :
∃ (q : ), x < q
theorem archimedean_iff_rat_lt {α : Type u_1}  :
∀ (x : α), ∃ (q : ), x < q
theorem archimedean_iff_rat_le {α : Type u_1}  :
∀ (x : α), ∃ (q : ), x q
theorem exists_rat_lt {α : Type u_1} [archimedean α] (x : α) :
∃ (q : ), q < x
theorem exists_rat_btwn {α : Type u_1} [archimedean α] {x y : α} (h : x < y) :
∃ (q : ), x < q q < y
theorem exists_nat_one_div_lt {α : Type u_1} [archimedean α] {ε : α} (hε : 0 < ε) :
∃ (n : ), 1 / (n + 1) < ε
theorem exists_pos_rat_lt {α : Type u_1} [archimedean α] {x : α} (x0 : 0 < x) :
∃ (q : ), 0 < q q < x
def round {α : Type u_1} [floor_ring α] (x : α) :

round rounds a number to the nearest integer. round (1 / 2) = 1

Equations
@[simp]
theorem round_zero {α : Type u_1} [floor_ring α] :
= 0
@[simp]
theorem round_one {α : Type u_1} [floor_ring α] :
= 1
theorem abs_sub_round {α : Type u_1} [floor_ring α] (x : α) :
abs (x - (round x)) 1 / 2
@[simp, norm_cast]
theorem rat.cast_floor {α : Type u_1} [floor_ring α] (x : ) :
@[simp, norm_cast]
theorem rat.cast_ceil {α : Type u_1} [floor_ring α] (x : ) :
@[simp, norm_cast]
theorem rat.cast_round {α : Type u_1} [floor_ring α] (x : ) :
=
theorem exists_rat_near {α : Type u_1} [archimedean α] (x : α) {ε : α} (ε0 : 0 < ε) :
∃ (q : ), abs (x - q) < ε
@[protected, instance]