Profinite number theory Hendrik Lenstra Mathematisch Instituut Universiteit Leiden Profinite number theory Hendrik Lenstra
The factorial number system Each n ∈ Z ≥ 0 has a unique representation ∞ � n = c i i ! with c i ∈ Z , i =1 0 ≤ c i ≤ i, # { i : c i � = 0 } < ∞ . In factorial notation: n = ( . . . c 3 c 2 c 1 ) ! . Profinite number theory Hendrik Lenstra
The factorial number system Each n ∈ Z ≥ 0 has a unique representation ∞ � n = c i i ! with c i ∈ Z , i =1 0 ≤ c i ≤ i, # { i : c i � = 0 } < ∞ . In factorial notation: n = ( . . . c 3 c 2 c 1 ) ! . Examples : 25 = (1001) ! , 1001 = (121221) ! . Note: c 1 ≡ n mod 2. Profinite number theory Hendrik Lenstra
Conversion Given n , one finds all c i by c 1 = (remainder of n 1 = n upon division by 2) , c i = (remainder of n i = n i − 1 − c i − 1 upon division by i +1) , i until n i = 0. Profinite number theory Hendrik Lenstra
Conversion Given n , one finds all c i by c 1 = (remainder of n 1 = n upon division by 2) , c i = (remainder of n i = n i − 1 − c i − 1 upon division by i +1) , i until n i = 0. Knowing c 1 , c 2 , . . . , c k − 1 is equivalent to knowing n modulo k !. Profinite number theory Hendrik Lenstra
Profinite numbers If one starts with n = − 1, one finds c i = i for all i : − 1 = ( . . . 54321) ! . In general, for a negative integer n one finds c i = i for almost all i . Profinite number theory Hendrik Lenstra
Profinite numbers If one starts with n = − 1, one finds c i = i for all i : − 1 = ( . . . 54321) ! . In general, for a negative integer n one finds c i = i for almost all i . A profinite integer is an infinite string ( . . . c 3 c 2 c 1 ) ! with each c i ∈ Z , 0 ≤ c i ≤ i . Notation: ˆ Z = { profinite integers } . Profinite number theory Hendrik Lenstra
A citizen of the world Features of ˆ Z : • it has an algebraic structure , • it comes with a topology , • it occurs in Galois theory , • it shows up in arithmetic geometry , • it connects to ultrafilters , • it carries “analytic” functions , • and it knows Fibonacci numbers ! Profinite number theory Hendrik Lenstra
Addition and multiplication For any k , the k last digits of n + m depend only on the k last digits of n and of m . Likewise for n · m . Profinite number theory Hendrik Lenstra
Addition and multiplication For any k , the k last digits of n + m depend only on the k last digits of n and of m . Likewise for n · m . Hence one can also define the sum and the product of any two profinite integers, and ˆ Z is a commutative ring . Profinite number theory Hendrik Lenstra
Ring homomorphisms Call a profinite integer ( . . . c 3 c 2 c 1 ) ! even if c 1 = 0 and odd if c 1 = 1. The map ˆ Z → Z / 2 Z , ( . . . c 3 c 2 c 1 ) ! �→ ( c 1 mod 2), is a ring homomorphism. Its kernel is 2 ˆ Z . Profinite number theory Hendrik Lenstra
Ring homomorphisms Call a profinite integer ( . . . c 3 c 2 c 1 ) ! even if c 1 = 0 and odd if c 1 = 1. The map ˆ Z → Z / 2 Z , ( . . . c 3 c 2 c 1 ) ! �→ ( c 1 mod 2), is a ring homomorphism. Its kernel is 2 ˆ Z . More generally, for any k ∈ Z > 0 , one has a ring homomorphism ˆ Z → Z /k ! Z sending ( . . . c 3 c 2 c 1 ) ! to i<k c i i ! mod k !), and it has kernel k ! ˆ ( � Z . Profinite number theory Hendrik Lenstra
Visualising profinite numbers Define v: ˆ Z → [0 , 1] by c i � v(( . . . c 3 c 2 c 1 ) ! ) = ( i + 1)! . i ≥ 1 Then v(2 ˆ Z ) = [0 , 1 2 ], v(1 + 2 ˆ Z ) = [ 1 2 , 1], v(1 + 6 ˆ Z ) = [ 1 2 , 2 3 ]. Profinite number theory Hendrik Lenstra
Visualising profinite numbers Define v: ˆ Z → [0 , 1] by c i � v(( . . . c 3 c 2 c 1 ) ! ) = ( i + 1)! . i ≥ 1 Then v(2 ˆ Z ) = [0 , 1 2 ], v(1 + 2 ˆ Z ) = [ 1 2 , 1], v(1 + 6 ˆ Z ) = [ 1 2 , 2 3 ]. One has #v − 1 r = 2 for r ∈ Q ∩ (0 , 1) , #v − 1 r = 1 for all other r ∈ [0 , 1] . Examples : v − 1 1 v − 1 2 v − 1 1 = {− 1 } . 2 = {− 2 , 1 } , 3 = {− 5 , 3 } , Profinite number theory Hendrik Lenstra
Graphs For graphical purposes, we represent a ∈ ˆ Z by v( a ) ∈ [0 , 1]. We visualise a function f : ˆ Z → ˆ Z by representing its graph { ( a, f ( a )) : a ∈ ˆ Z } in [0 , 1] × [0 , 1]. Profinite number theory Hendrik Lenstra
Illustration by Willem Jan Palenstijn Profinite number theory Hendrik Lenstra
Four functions In green: the graph of a �→ a . In blue: the graph of a �→ − a . In yellow: the graph of a �→ a − 1 − 1 ( a ∈ ˆ Z ∗ ). In orange/red/brown: the graph of a �→ F ( a ), the “ a -th Fibonacci number”. Profinite number theory Hendrik Lenstra
A formal definition A more satisfactory definition is ∞ � ˆ Z = { ( a n ) ∞ n =1 ∈ ( Z /n Z ) : n | m ⇒ a m ≡ a n mod n } . n =1 This is a subring of � ∞ n =1 ( Z /n Z ). Z ∗ is a subgroup of � ∞ Its unit group ˆ n =1 ( Z /n Z ) ∗ . Profinite number theory Hendrik Lenstra
A formal definition A more satisfactory definition is ∞ � ˆ Z = { ( a n ) ∞ n =1 ∈ ( Z /n Z ) : n | m ⇒ a m ≡ a n mod n } . n =1 This is a subring of � ∞ n =1 ( Z /n Z ). Z ∗ is a subgroup of � ∞ Its unit group ˆ n =1 ( Z /n Z ) ∗ . Alternative definition: ˆ Z = End( Q / Z ), the endomorphism Z ∗ = Aut( Q / Z ). ring of the abelian group Q / Z . Then ˆ Profinite number theory Hendrik Lenstra
Basic facts The ring ˆ Z is uncountable , it is commutative , and it has Z as a subring. It has lots of zero-divisors. Profinite number theory Hendrik Lenstra
Basic facts The ring ˆ Z is uncountable , it is commutative , and it has Z as a subring. It has lots of zero-divisors. For each m ∈ Z > 0 , there is a ring homomorphism ˆ a = ( a n ) ∞ Z → Z /m Z , n =1 �→ a m , which together with the group homomorphism ˆ Z → ˆ Z , a �→ ma , fits into a short exact sequence m 0 → ˆ → ˆ − Z → Z /m Z → 0 . Z Profinite number theory Hendrik Lenstra
Profinite rationals Write ∞ � Q = { ( a n ) ∞ ˆ n =1 ∈ ( Q /n Z ) : n | m ⇒ a m ≡ a n mod n Z } . n =1 The additive group ˆ Q has exactly one ring multiplication extending the ring multiplication on ˆ Z . Profinite number theory Hendrik Lenstra
Profinite rationals Write ∞ � Q = { ( a n ) ∞ ˆ n =1 ∈ ( Q /n Z ) : n | m ⇒ a m ≡ a n mod n Z } . n =1 The additive group ˆ Q has exactly one ring multiplication extending the ring multiplication on ˆ Z . It is a commutative ring, with Q and ˆ Z as subrings, and Z ∼ Q = Q + ˆ ˆ Z = Q · ˆ = Q ⊗ Z ˆ Z (as rings). Profinite number theory Hendrik Lenstra
Topology If each Z /n Z has the discrete topology and � ∞ n =1 ( Z /n Z ) the product topology, then ˆ Z is closed in � ∞ n =1 ( Z /n Z ). Profinite number theory Hendrik Lenstra
Topology If each Z /n Z has the discrete topology and � ∞ n =1 ( Z /n Z ) the product topology, then ˆ Z is closed in � ∞ n =1 ( Z /n Z ). One can define the topology on ˆ Z by the metric 1 d( x, y ) = min { k ∈ Z > 0 : x �≡ y mod ( k + 1)! } 1 = min { k ∈ Z > 0 : c k � = d k } if x = ( . . . c 3 c 2 c 1 ) ! , y = ( . . . d 3 d 2 d 1 ) ! , x � = y . Profinite number theory Hendrik Lenstra
More topology Fact : ˆ Z is a compact Hausdorff totally disconnected topological ring. One can make the map v: ˆ Z → [0 , 1] into a homeomorphism by “cutting” [0 , 1] at every r ∈ Q ∩ (0 , 1). Profinite number theory Hendrik Lenstra
More topology Fact : ˆ Z is a compact Hausdorff totally disconnected topological ring. One can make the map v: ˆ Z → [0 , 1] into a homeomorphism by “cutting” [0 , 1] at every r ∈ Q ∩ (0 , 1). A neighborhood base of 0 in ˆ Z is { m ˆ Z : m ∈ Z > 0 } . With the same neighborhood base, ˆ Q is also a topological ring. It is locally compact, Hausdorff, and totally disconnected. Profinite number theory Hendrik Lenstra
Amusements for algebraists We have ˆ Z ⊂ A = � ∞ n =1 ( Z /n Z ). Z ∼ Theorem. One has A / ˆ = A as additive topological groups. Proof (Carlo Pagano): write down a surjective continuous group homomorphism ǫ : A → A with ker ǫ = ˆ Z . Profinite number theory Hendrik Lenstra
Amusements for algebraists We have ˆ Z ⊂ A = � ∞ n =1 ( Z /n Z ). Z ∼ Theorem. One has A / ˆ = A as additive topological groups. Proof (Carlo Pagano): write down a surjective continuous group homomorphism ǫ : A → A with ker ǫ = ˆ Z . Theorem. One has A ∼ = A × ˆ Z as groups but not as topological groups. Here the axiom of choice comes in. Profinite number theory Hendrik Lenstra
Profinite groups In infinite Galois theory, the Galois groups that one encounters are profinite groups . A profinite group is a topological group that is isomorphic to a closed subgroup of a product of finite discrete groups. Equivalent definition: it is a compact Hausdorff totally disconnected topological group. Examples : the additive group of ˆ Z and its unit group ˆ Z ∗ are profinite groups. Profinite number theory Hendrik Lenstra
ˆ Z as the analogue of Z Familiar fact. For each group G and each γ ∈ G there is a unique group homomorphism Z → G with 1 �→ γ , namely n �→ γ n . Analogue for ˆ Z . For each profinite group G and each γ ∈ G there is a unique group homomorphism ˆ Z → G with 1 �→ γ , and it is continuous. Notation: a �→ γ a . Profinite number theory Hendrik Lenstra
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