correlation immune functions and learning

Correlation Immune Functions and Learning

Lisa Hellerstein

Polytechnic Institute of NYU

Brooklyn, NY

Includes joint work with Bernard Rosell (AT&T), Eric Bach and David Page (U. of Wisconsin), and Soumya Ray (Case

Western)

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Identifying relevant variables from random examples

107210...21 ),,,( xxxxxxf

x f(x)(1,1,0,0,0,1,1,0,1,0) 1(0,1,0,0,1,0,1,1,0,1) 1(1,0,0,1,0,1,0,0,1,0) 0

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Technicalities

• Assume random examples drawn from uniform distribution over {0,1}n

• Have access to source of random examples

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Detecting that a variable is relevant

• Look for dependence between input variables and output

If xi irrelevant P(f=1|xi=1) = P(f=1|xi=0)

If xi relevant P(f=1|xi=1) ≠ P(f=1|xi=0)

for previous function f

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Unfortunately…

),,(parity),,,( 107210...21 xxxxxxf

xi relevant P(f=1|xi=1) = 1/2 = P(f=1|xi=0) xi irrelevant P(f=1|xi=1) = 1/2 = P(f=1|xi=0)

Finding a relevant variable easy for some functions.

Not so easy for others.

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How to find the relevant variables

• Suppose you know r (# of relevant vars)Assume r << n

(Think of r = log n)

• Get m random examples, where

m = poly(2r ,log n,1/δ)

• With probability > 1-δ, have enough info to determine which r variables are relevant– All other sets of r variables can be ruled out

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x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 f(1, 1, 0, 1, 1, 0, 1, 0, 1, 0) 1(0, 1, 1, 1, 1, 0, 1, 1, 0, 0) 0(1, 1, 1, 0, 0, 0, 0, 0, 0, 0) 1(0, 0, 0, 1, 1, 0, 0, 0, 0, 0) 0(1, 1, 1, 0, 0, 0, 1, 1, 1, 1) 0

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x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 f(1, 1, 0, 1, 1, 0, 1, 0, 1, 0) 1(0, 1, 1, 1, 1, 0, 1, 1, 0, 0) 0(1, 1, 1, 0, 0, 0, 0, 0, 0, 0) 1(0, 0, 0, 1, 1, 0, 0, 0, 0, 0) 0(1, 1, 1, 0, 0, 0, 1, 1, 0, 1) 0

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x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 f(1, 1, 0, 1, 1, 0, 1, 0, 1, 0) 1(0, 1, 1, 1, 1, 0, 1, 1, 0, 0) 0(1, 1, 1, 0, 0, 0, 0, 0, 0, 0) 1(0, 0, 0, 1, 1, 0, 0, 0, 0, 0) 0(1, 1, 1, 0, 0, 0, 1, 1, 0, 1) 0

x3, x5, x9 can’t be the relevant variables

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x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 f(1, 1, 0, 1, 1, 0, 1, 0, 1, 0) 1(0, 1, 1, 1, 1, 0, 1, 1, 0, 0) 0(1, 1, 1, 0, 0, 0, 0, 0, 0, 0) 1(0, 0, 0, 1, 1, 0, 0, 0, 0, 0) 0(1, 1, 1, 0, 0, 0, 1, 1, 1, 1) 0

x1, x3, x10 ok

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• Naïve algorithm: Try all combinations of r variables. Time ≈ nr

• Mossel, O’Donnell, Servedio [STOC 2003]– Algorithm that takes time ≈ ncr where c ≈ .704– Subroutine: Find a single relevant variable

Still open: Can this bound be improved?

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If output of f is dependent on xi, can detect dependence (whp) in time poly(n, 2r) and identify xi as relevant.

Problematic Functions

Every variable is independent of output of f P[f=1|xi=0] = P[f=1|xi=1] for all xi

Equivalently, all degree 1 Fourier coeffs = 0

Functions with this property said to be CORRELATION-IMMUNE

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P[f=1|xi=0] = P[f=1|xi=1] for all xi

Geometrically:

00 01

10 11

e.g. n=2

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P[f=1|xi=0] = P[f=1|xi=1] for all xi

Geometrically:

00 01

10 111

10

0

Parity(x1,x2)

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P[f=1|xi=0] = P[f=1|xi=1] for all xi

Geometrically:

00 01

10 111

10

0

X1=1

X1=0

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P[f=1|xi=0] = P[f=1|xi=1] for all xi

00 01

10 111

10

0X2=0 X2=1

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• Other correlation-immune functions besides parity?– f(x1,…,xn) = 1 iff x1 = x2 = … = xn

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• Other correlation-immune functions besides parity?– All reflexive functions

xallfor )xf(f(x)

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• Other correlation-immune functions besides parity?– All reflexive functions

– More…

xallfor )xf(f(x)

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Correlation-immune functions and decision tree learners

• Decision tree learners in ML– Popular machine learning approach (CART, C4.5)– Given set of examples of Boolean function, build a

decision tree

• Heuristics for decision tree learning– Greedy, top-down– Differ in way choose which variable to put in node– Pick variable having highest “gain”– P[f=1|xi=1] = P[f=1|xi=0] means 0 gain

• Correlation-immune functions problematic for decision tree learners

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• Lookahead

• Skewing: An efficient alternative to lookahead for decision tree induction. IJCAI 2003 [Page, Ray]

• Why skewing works: learning difficult Boolean functions with greedy tree learners. ICML 2005 [Rosell, Hellerstein, Ray, Page]

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StoryPart One

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• How many difficult functions?

• More than

n

# fns

0 1 2 3 4 5

2 2 4 18 648 3140062

n-1 22

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• How many different hard functions?

• More than

SOMEONE MUST HAVE STUDIED THESE FUNCTIONS BEFORE…

n

# fns

0 1 2 3 4 5

2 2 4 18 648 3140062

n/2 22

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26

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StoryPart Two

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• I had lunch with Eric Bach

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Roy, B. K. 2002. A Brief Outline of Research on Correlation Immune Functions. In Proceedings of the 7th Australian Conference on information Security and Privacy (July 03 - 05, 2002). L. M. Batten and J. Seberry, Eds. Lecture Notes In Computer Science, vol. 2384. Springer-Verlag, London, 379-394.

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Correlation-immune functions• k-correlation immune function

– For every subset S of the input variables s.t. 1 ≤ |S| ≤ k

P[f | S] = P[f]– [Xiao, Massey 1988] Equivalently, all Fourier

coefficients of degree i are 0, for 1 ≤ i ≤ k

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Siegenthaler’s Theorem

If f is k-correlation immune, then the GF[2] polynomial for f has degree at most n-k.

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Siegenthaler’s Theorem [1984]

If f is k-correlation immune, then the GF[2] polynomial for f has degree at most n-k.

Algorithm of Mossel, O’Donnell, Servedio [STOC 2003] based on this theorem

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End of Story

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Non-uniform distributions• Correlation-immune functions are defined

wrt the uniform distribution

• What if distribution is biased?

e.g. each bit 1 with probability ¾

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f(x1,x2) = parity(x1,x2)each bit 1 with probability 3/4

x parity(x) P[x]

00 0 1/16

01 1 3/16

10 1 3/16

11 0 9/16

P[f=1|x1=1] ≠ P[f=1|x1=0]

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f(x1,x2) = parity(x1,x2)p=1 with probability 1/4

x parity(x) P[x]

00 0 1/16

01 1 3/16

10 1 3/16

11 0 9/16

P[f=1|x1=1] ≠ P[f=1|x1=0]

For added irrelevant variables, would be equal

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Correlation-immunity wrt p-biased distributions

Definitions• f is correlation-immune wrt distribution D if PD[f=1|xi=1] = PD[f=1|xi=0]for all xi• p-biased distribution Dp: each bit set to 1

independently with probability p– For all p-biased distributions D,

PD[f=1|xi=1] = PD[f=1|xi=0] for all irrelevant xi

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Lemma: Let f(x1,…,xn) be a Boolean function with r relevant variables. Then f is correlation immune w.r.t. Dp for at most r-1 values of p.

Pf: Correlation immune wrt Dp means P[f=1|xi=1] – P[f=1|xi=0] = 0 (*)for all xi.Consider fixed f and xi. Can write lhs of (*)as polynomial h(p).

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• e.g. f(x1,x2, x3) = parity(x1,x2, x3)p-biased distribution Dph(p) = PDp[f=1|x1=1] - PDp[f=1|x1=0] =

( p2 + p(1-p) ) – ( p(1-p) + (1-p)p )

If add irrelevant variable, this polynomial doesn’t change

• h(p) for arbitrary f, variable xi, has degree <= r-1, where r is number of variables.

• f correlation-immune wrt at most r-1 values of p, unless h(p) identically 0 for all xi.

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h(p) = PDp[f=1|xi=1] -PDp[f=1|xi=0]

where wd is number of inputs x for which f(x)=1, xi=1, and x contains exactly d additional 1’s.

i.e. wd = number of positive assignments of fxi<-1 of Hamming weight d

• Similar expression for PDp[f=1|xi=0]

1n

0d

d1nddiDp p)(1pw1]x|1[fP

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PDp[f=1|xi=1] - PDp[f=1|xi=0] =

where wd = number of positive assignments of fxi<-1 of Hamming weight d

rd = number of positive assignments of fxi<-0 of Hamming weight d

Not identically 0 iff wd ≠ rd for some d

1

0

1)1()(]1|1[n

d

dndddiDp pprwxfP

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Property of Boolean functions

Lemma: If f has at least one relevant variable, then for some relevant variable xi, and some d,

wd ≠ rd for some dwhere

wd = number of positive assignments of fxi<-1 of Hamming weight d

rd = number of positive assignments of fxi<-0 of Hamming weight d

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How much does it help to have access to examples from different distributions?

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How much does it help to have access to examples from different distributions?

Hellerstein, Rosell, Bach, Page, Ray

Exploiting Product Distributions to Identify Relevant Variables of Correlation Immune Functions

Exploiting Product Distributions to Identify Relevant Variables of Correlation Immune

Functions [Hellerstein, Rosell, Bach, Ray, Page]

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• Even if f is not correlation-immune wrt Dp, may need very large sample to detect relevant variable– if value of p very near root of h(p)

• Lemma: If h(p) not identically 0, then for some value of p in the set

{ 1/(r+1),2/(r+1),3/(r+1)…, (r+1)/(r+1) },

h(p) ≥ 1/(r+1)r-1

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• Algorithm to find a relevant variable– Uses examples from distributions Dp, for

p = 1/(r+1),2/(r+1),3/(r+1)…, (r+1)/(r+1)– sample size poly((r+1) r, log n, log 1/δ)

[Essentially same algorithm found independently by Arpe and Mossel, using very different techniques]

• Another algorithm to find a relevant variable– Based on proving (roughly) that if choose random p,

then h2(p) likely to be reasonably large. Uses prime number theorem.

– Uses examples from poly(2r, log 1/ δ) distributions Dp.– Sample size poly(2r, log n, log 1/ δ)

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Better algorithms?

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Summary

• Finding relevant variables (junta-learning)

• Correlation-immune functions

• Learning from p-biased distributions

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Moral of the Story

• Handbook of integer sequences can be useful in doing literature search

• Eating lunch with the right person can be much more useful