Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Subgroups of the multiplicative group Greg Martin University of British Columbia joint work with Lee Troupe Analytic Number Theory 2017 CMS Winter Meeting University of Waterloo December 10, 2017 slides can be found on my web page www.math.ubc.ca/ ∼ gerg/index.shtml?slides The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Outline Background, and distribution of ( φ -)additive functions 1 Results on the distribution of the number of subgroups of Z × 2 n Outline of the proofs 3 The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Our objects of study Definition The “multiplicative group” (or unit group) modulo n is Z × n = ( Z / n Z ) × , the group of reduced residue classes under multiplication (mod n ) . Z × n is some finite abelian group with φ ( n ) elements (usually not cyclic). Questions about its structure often turn into number theory (example: its exponent is the Carmichael λ -function). Overarching question (I heard it from Shparlinski) How many subgroups does Z × n usually have? Notation (used throughout the talk) I ( n ) is the number of isomorphism classes of subgroups of Z × n . G ( n ) is the number of subsets of Z × n that are subgroups (that is, subgroups not up to isomorphism). The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Our objects of study Definition The “multiplicative group” (or unit group) modulo n is Z × n = ( Z / n Z ) × , the group of reduced residue classes under multiplication (mod n ) . Z × n is some finite abelian group with φ ( n ) elements (usually not cyclic). Questions about its structure often turn into number theory (example: its exponent is the Carmichael λ -function). Overarching question (I heard it from Shparlinski) How many subgroups does Z × n usually have? Notation (used throughout the talk) I ( n ) is the number of isomorphism classes of subgroups of Z × n . G ( n ) is the number of subsets of Z × n that are subgroups (that is, subgroups not up to isomorphism). The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Our objects of study Definition The “multiplicative group” (or unit group) modulo n is Z × n = ( Z / n Z ) × , the group of reduced residue classes under multiplication (mod n ) . Z × n is some finite abelian group with φ ( n ) elements (usually not cyclic). Questions about its structure often turn into number theory (example: its exponent is the Carmichael λ -function). Overarching question (I heard it from Shparlinski) How many subgroups does Z × n usually have? Notation (used throughout the talk) I ( n ) is the number of isomorphism classes of subgroups of Z × n . G ( n ) is the number of subsets of Z × n that are subgroups (that is, subgroups not up to isomorphism). The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Our objects of study Definition The “multiplicative group” (or unit group) modulo n is Z × n = ( Z / n Z ) × , the group of reduced residue classes under multiplication (mod n ) . Z × n is some finite abelian group with φ ( n ) elements (usually not cyclic). Questions about its structure often turn into number theory (example: its exponent is the Carmichael λ -function). Overarching question (I heard it from Shparlinski) How many subgroups does Z × n usually have? Notation (used throughout the talk) I ( n ) is the number of isomorphism classes of subgroups of Z × n . G ( n ) is the number of subsets of Z × n that are subgroups (that is, subgroups not up to isomorphism). The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Distribution results: different strengths By way of analogy: some historical results about the distribution of ω ( n ) , the number of distinct prime factors of n . The average value of ω ( n ) is log log n . — requires an asymptotic formula for � n ≤ x ω ( n ) The normal order (typical size) of ω ( n ) is log log n . � 2 � — requires estimate for variance � ω ( n ) − log log n n ≤ x Erd˝ os–Kac theorem: ω ( n ) is asymptotically distributed like a normal random variable with mean log log n and variance log log n . (More precise statement on next slide.) — requires asymptotic formulas for all central moments � k � � ω ( n ) − log log n n ≤ x The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Distribution results: different strengths By way of analogy: some historical results about the distribution of ω ( n ) , the number of distinct prime factors of n . The average value of ω ( n ) is log log n . — requires an asymptotic formula for � n ≤ x ω ( n ) The normal order (typical size) of ω ( n ) is log log n . � 2 � — requires estimate for variance � ω ( n ) − log log n n ≤ x Erd˝ os–Kac theorem: ω ( n ) is asymptotically distributed like a normal random variable with mean log log n and variance log log n . (More precise statement on next slide.) — requires asymptotic formulas for all central moments � k � � ω ( n ) − log log n n ≤ x The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Distribution results: different strengths By way of analogy: some historical results about the distribution of ω ( n ) , the number of distinct prime factors of n . The average value of ω ( n ) is log log n . — requires an asymptotic formula for � n ≤ x ω ( n ) The normal order (typical size) of ω ( n ) is log log n . � 2 � — requires estimate for variance � ω ( n ) − log log n n ≤ x Erd˝ os–Kac theorem: ω ( n ) is asymptotically distributed like a normal random variable with mean log log n and variance log log n . (More precise statement on next slide.) — requires asymptotic formulas for all central moments � k � � ω ( n ) − log log n n ≤ x The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Distribution results: different strengths By way of analogy: some historical results about the distribution of ω ( n ) , the number of distinct prime factors of n . The average value of ω ( n ) is log log n . — requires an asymptotic formula for � n ≤ x ω ( n ) The normal order (typical size) of ω ( n ) is log log n . � 2 � — requires estimate for variance � ω ( n ) − log log n n ≤ x Erd˝ os–Kac theorem: ω ( n ) is asymptotically distributed like a normal random variable with mean log log n and variance log log n . (More precise statement on next slide.) — requires asymptotic formulas for all central moments � k � � ω ( n ) − log log n n ≤ x The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Erd˝ os–Kac laws Definition A function f ( n ) satisfies an Erd˝ os–Kac law with mean µ ( n ) and variance σ 2 ( n ) if � u � � 1 n ≤ x : f ( n ) − µ ( n ) 1 e − t 2 / 2 dt x # < u = √ lim σ ( n ) x →∞ 2 π −∞ for every real number u . Standard notation ω ( n ) is the number of distinct prime factors of n . Ω( n ) is the number of prime factors of n counted with multiplicity. Theorem (Erd˝ os–Kac, 1940) Both ω ( n ) and Ω( n ) satisfy Erd˝ os–Kac laws with mean log log n and variance log log n . The distribution of the number of subgroups of the multiplicative group Greg Martin
Background, ( φ -)additive functions Results on I ( n ) and G ( n ) Outline of the proofs Erd˝ os–Kac laws Definition A function f ( n ) satisfies an Erd˝ os–Kac law with mean µ ( n ) and variance σ 2 ( n ) if � u � � 1 n ≤ x : f ( n ) − µ ( n ) 1 e − t 2 / 2 dt x # < u = √ lim σ ( n ) x →∞ 2 π −∞ for every real number u . Standard notation ω ( n ) is the number of distinct prime factors of n . Ω( n ) is the number of prime factors of n counted with multiplicity. Theorem (Erd˝ os–Kac, 1940) Both ω ( n ) and Ω( n ) satisfy Erd˝ os–Kac laws with mean log log n and variance log log n . The distribution of the number of subgroups of the multiplicative group Greg Martin
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