To Infinity And Beyond

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To Infinity And Beyond!
Great Theoretical Ideas In Computer Science
CS 15-251
Lecture 11
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The Ideal Computerno bound on amount of memory

• Whenever you run out of memory, the computer
  contacts the factory. A maintenance person is
  flown by helicopter and attaches 100 Gig of RAM
  and all programs resume their computations, as if
  they had never been interrupted.

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An Ideal Computer Can Be Programmed To Print Out

• ? 3.14159265358979323846264
• 2 2.0000000000000000000000
• e 2.7182818284559045235336
• 1/3 0.33333333333333333333.
• ? 1.6180339887498948482045

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Computable Real Numbers

• A real number r is computable if there is a
  program that prints out the decimal
  representation of r from left to right. Thus,
  each digit of r will eventually be printed as
  part of an infinite sequence.

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Describable Numbers

• A real number r is describable if it can be
  unambiguously denoted by a finite piece of
  English text.
• 2 Two.
• ? The area of a circle of radius one.

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Theorem Every computable real is also describable

• Proof Let r be a computable real that is output
  by a program P. The following is an unambiguous
  denotation
• The real number output byP

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MORAL A computer program can be viewed as a
description of its output.
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Are all real numbers describable?
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To INFINITY . and Beyond!
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Correspondence Principle

• If two finite sets can be placed into 1-1 onto
  correspondence, then they have the same size.

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Correspondence Definition

• Two finite sets are defined to have the same
  size if and only if they can be placed into 1-1
  onto correspondence.

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Georg Cantor (1845-1918)

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Cantors Definition (1874)

• Two sets are defined to have the same size if and
  only if they can be placed into 1-1 onto
  correspondence.

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Cantors Definition (1874)

• Two sets are defined to have the same cardinality
  if and only if they can be placed into 1-1 onto
  correspondence.

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Do N and E have the same cardinality?

• N 0, 1, 2, 3, 4, 5, 6, 7, .
• E The even, natural numbers.

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• E and N do not have the same cardinality! E is a
  proper subset of N with plenty left over.
• The attempted correspondence f(x)x does not take
  E onto N.

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E and N do have the same cardinality! 0, 1, 2,
3, 4, 5, .0, 2, 4, 6, 8,10, . f(x) 2x
is 1-1 onto.
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Lesson Cantors definition only requires that
some 1-1 correspondence between the two sets is
onto, not that all 1-1 correspondences are onto.
This distinction never arises when the sets are
finite.
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If this makes you feel uncomfortable..
TOUGH! It is the price that you must pay to
reason about infinity
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Do N and Z have the same cardinality?

• N 0, 1, 2, 3, 4, 5, 6, 7, .
• Z , -2, -1, 0, 1, 2, 3, .

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N and Z do have the same cardinality! 0, 1, 2,
3, 4, 5, 6 0, 1, -1, 2, -2, 3, -3, . f(x)
?x/2? if x is odd -x/2 if x is
even
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Transitivity Lemma

• If f A-gtB 1-1 onto, and g B-gtC 1-1 onto
• Then h(x) g(f(x)) is 1-1 onto A-gtC
• Hence, N, E, and Z all have the same cardinality.

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Do N and Q have the same cardinality?

• 0, 1, 2, 3, 4, 5, 6, 7, .
• Q The Rational Numbers

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No way! The rationals are dense between any two
there is a third. You cant list them one by one
without leaving out an infinite number of them.
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Dont jump to conclusions! There is a clever way
to list the rationals, one at a time, without
missing a single one!
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The point at x,y represents x/y
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3
0
1
2
The point at x,y represents x/y
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We call a set countable if it can be placed into
1-1 onto correspondence with the natural
numbers. So far we know that N, E, Z, and Q are
countable.
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Do N and R have the same cardinality?

• N 0, 1, 2, 3, 4, 5, 6, 7, .
• R The Real Numbers

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No way! You will run out of natural numbers long
before you match up every real.
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Dont jump to conclusions! You cant be sure that
there isnt some clever correspondence that you
havent thought of yet.
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I am sure! Cantor proved it. He invented a very
important technique called DIAGONALIZATION.
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Theorem The set I of reals between 0 and 1 is
not countable.

• Proof by contradiction
• Suppose I is countable. Let f be the 1-1 onto
  function from N to I. Make a list L as follows
• 0 decimal expansion of f(0)1 decimal expansion
  of f(1)
• k decimal expansion of f(k)

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Theorem The set I of reals between 0 and 1 is
not countable.

• Proof by contradiction
• Suppose I is countable. Let f be the 1-1 onto
  function from N to I. Make a list L as follows
• 0 .33333333333333333333331 .314159265657839593
  8594982..
• k .345322214243555345221123235..

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ConfuseL . C0 C1 C2 C3 C4
C5
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5, if dk6 6, otherwise
Ck
ConfuseL . C0 C1 C2 C3 C4
C5
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5, if dk6 6, otherwise
Ck
. C0 C1 C2 C3 C4 C5
By design, ConfuseL cant be on the list!
ConfuseL differs from the kth element on the list
in the kth position. Contradiction of assumption
that list is complete.
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The set of reals is uncountable!
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Hold it! Why cant the same argument be used to
show that Q is uncountable?
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The argument works the same for Q until the
punchline. CONFUSEL is not necessarily rational,
so there is no contradiction from the fact that
it is missing.
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Standard Notation

• Any finite alphabet
• Example a,b,c,d,e,,z
• S All finite strings of symbols
  from S including the empty string e

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Theorem Every infinite subset S of S is
countable

• Proof List S in alphabetical order. Map the
  first word to 0, the second to 1, and so on.

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Stringing Symbols Together

• The symbols on a standard
  keyboard
• The set of all possible Java programs is a subset
  of S
• The set of all possible finite pieces of English
  text is a subset of S

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Thus The set of all possible Java programs is
countable. The set of all possible finite length
pieces of English text is countable.
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There are countably many Java program and
uncountably many reals. HENCE MOST REALS ARE
NOT COMPUTABLE.
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There are countably many descriptions and
uncountably many reals. Hence MOST REAL
NUMBERS ARE NOT DESCRIBEABLE!
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BINGO BONZO!
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Is there a real number that can be described, but
not computed?
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We know there are at least 2 infinities. Are
there more?
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There are many, many, many, many, many more! So
many infinities that the number of infinities
goes beyond any infinity!
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Power Set

• The power set of S is the set of all subsets of
  S. The power set is denoted P(S).
• Proposition If S is finite, the power set of S
  has cardinality 2S

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Theorem S cant be put into 1-1 correspondence
with P(S)

• Suppose fS-gtP(S) is 1-1 and ONTO.
• Let CONFUSE All x in S such that x is not
  contained in f(x)
• There is some y such that f(y)CONFUSE
• IS Y in CONFUSE?
• YES definition of CONFUSE implies NO
• NO definition of CONFUSE implies YES
• CONTRADICTION

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This proves that there are at least a countable
number of infinities. The first infinity is
called ?0
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?0,, ?1, ?2, .. Cantor wanted to show that the
number of reals was ?1
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Cantor couldnt prove that ?1 was the number of
reals. This helped feed his depression. He called
it The Continuum Hypothesis.
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The Continuum Hypothesis cant be proved or
disproved! This has been proved!
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How Many Infinities?

• Suppose there are ?q infinities.
• For all i, let Si be a set of size ??i.
• S union of Si for i ? ?q
• Easy to prove that S is bigger than ??q
• Contradiction