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Problem: To Catch a Lion in the Sahara Desert.

1. Mathematical Methods

1.1 The Hilbert (axiomatic) method

We place a locked cage onto a given point in the desert. After that

we introduce the following logical system:

Axiom 1: The set of lions in the Sahara is not empty.

Axiom 2: If there exists a lion in the Sahara, then there exists a

lion in the cage.

Procedure: If P is a theorem, and if the following is holds:

"P implies Q", then Q is a theorem.

Theorem 1: There exists a lion in the cage.

1.2 The geometrical inversion method

We place a spherical cage in the desert, enter it and lock it from

inside. We then performe an inversion with respect to the cage. Then

the lion is inside the cage, and we are outside.

1.3 The projective geometry method

Without loss of generality, we can view the desert as a plane surface.

We project the surface onto a line and afterwards the line onto an

interiour point of the cage. Thereby the lion is mapped onto that same

point.

1.4 The Bolzano-Weierstrass method

Divide the desert by a line running from north to south. The lion is

then either in the eastern or in the western part. Let's assume it is

in the eastern part. Divide this part by a line running from east to

west. The lion is either in the northern or in the southern part.

Let's assume it is in the northern part. We can continue this process

arbitrarily and thereby constructing with each step an increasingly

narrow fence around the selected area. The diameter of the chosen

partitions converges to zero so that the lion is caged into a fence of

arbitrarily small diameter.

1.5 The set theoretical method

We observe that the desert is a separable space. It therefore

contains an enumerable dense set of points which constitutes a

sequence with the lion as its limit. We silently approach the lion in

this sequence, carrying the proper equipment with us.

1.6 The Peano method

In the usual way construct a curve containing every point in the

desert. It has been proven [1] that such a curve can be traversed in

arbitrarily short time. Now we traverse the curve, carrying a spear,

in a time less than what it takes the lion to move a distance equal to

its own length.

1.7 A topological method

We observe that the lion possesses the topological gender of a torus.

We embed the desert in a four dimensional space. Then it is possible

to apply a deformation [2] of such a kind that the lion when returning

to the three dimensional space is all tied up in itself. It is then

completely helpless.

1.8 The Cauchy method

We examine a lion-valued function f(z). Be \zeta the cage. Consider

the integral

1 [ f(z)

------- I --------- dz

2 \pi i ] z - \zeta

C

where C represents the boundary of the desert. Its value is f(zeta),

i.e. there is a lion in the cage [3].

1.9 The Wiener-Tauber method

We obtain a tame lion, L_0, from the class L(-\infinity,\infinity),

whose fourier transform vanishes nowhere. We put this lion somewhere

in the desert. L_0 then converges toward our cage. According to the

general Wiener-Tauner theorem [4] every other lion L will converge

toward the same cage. (Alternatively we can approximate L arbitrarily

close by translating L_0 through the desert [5].)

2 Theoretical Physics Methods

2.1 The Dirac method

We assert that wild lions can ipso facto not be observed in the Sahara

desert. Therefore, if there are any lions at all in the desert, they

are tame. We leave catching a tame lion as an execise to the reader.

2.2 The Schroedinger method

At every instant there is a non-zero probability of the lion being in

the cage. Sit and wait.

2.3 The nuclear physics method

Insert a tame lion into the cage and apply a Majorana exchange

operator [6] on it and a wild lion.

As a variant let us assume that we would like to catch (for argument's

sake) a male lion. We insert a tame female lion into the cage and

apply the Heisenberg exchange operator [7], exchanging spins.

2.4 A relativistic method

All over the desert we distribute lion bait containing large amounts

of the companion star of Sirius. After enough of the bait has been

eaten we send a beam of light through the desert. This will curl

around the lion so it gets all confused and can be approached without

danger.

3 Experimental Physics Methods

3.1 The thermodynamics method

We construct a semi-permeable membrane which lets everything but lions

pass through. This we drag across the desert.

3.2 The atomic fission method

We irradiate the desert with slow neutrons. The lion becomes

radioactive and starts to disintegrate. Once the disintegration

process is progressed far enough the lion will be unable to resist.

3.3 The magneto-optical method

We plant a large, lense shaped field with cat mint (nepeta cataria)

such that its axis is parallel to the direction of the horizontal

component of the earth's magnetic field. We put the cage in one of the

field's foci. Throughout the desert we distribute large amounts of

magnetized spinach (spinacia oleracea) which has, as everybody knows,

a high iron content. The spinach is eaten by vegetarian desert

inhabitants which in turn are eaten by the lions. Afterwards the

lions are oriented parallel to the earth's magnetic field and the

resulting lion beam is focussed on the cage by the cat mint lense.

[1] After Hilbert, cf. E. W. Hobson, "The Theory of Functions of a Real

Variable and the Theory of Fourier's Series" (1927), vol. 1, pp 456-457

[2] H. Seifert and W. Threlfall, "Lehrbuch der Topologie" (1934), pp 2-3

[3] According to the Picard theorem (W. F. Osgood, Lehrbuch der

Funktionentheorie, vol 1 (1928), p 178) it is possible to catch every lion

except for at most one.

[4] N. Wiener, "The Fourier Integral and Certain of itsl Applications" (1933),

pp 73-74

[5] N. Wiener, ibid, p 89

[6] cf e.g. H. A. Bethe and R. F. Bacher, "Reviews of Modern Physics", 8

(1936), pp 82-229, esp. pp 106-107

[7] ibid

--

4 Contributions from Computer Science.

4.1 The search method

We assume that the lion is most likely to be found in the direction to

the north of the point where we are standing. Therefore the REAL

problem we have is that of speed, since we are only using a PC to

solve the problem.

4.2 The parallel search method.

By using parallelism we will be able to search in the direction to the

north much faster than earlier.

4.3 The Monte-Carlo method.

We pick a random number indexing the space we search. By excluding

neighboring points in the search, we can drastically reduce the number

of points we need to consider. The lion will according to probability

appear sooner or later.

4.4 The practical approach.

We see a rabbit very close to us. Since it is already dead, it is

particularly easy to catch. We therefore catch it and call it a lion.

4.5 The common language approach.

If only everyone used ADA/Common Lisp/Prolog, this problem would be

trivial to solve.

4.6 The standard approach.

We know what a Lion is from ISO 4711/X.123. Since CCITT have specified

a Lion to be a particular option of a cat we will have to wait for a

harmonized standard to appear. $20,000,000 have been funded for

initial investigastions into this standard development.

4.7 Linear search.

Stand in the top left hand corner of the Sahara Desert. Take one step

east. Repeat until you have found the lion, or you reach the right

hand edge. If you reach the right hand edge, take one step

southwards, and proceed towards the left hand edge. When you finally

reach the lion, put it the cage. If the lion should happen to eat you

before you manage to get it in the cage, press the reset button, and

try again.

4.8 The Dijkstra approach:

The way the problem reached me was: catch a wild lion in the Sahara

Desert. Another way of stating the problem is:

Axiom 1: Sahara elem deserts

Axiom 2: Lion elem Sahara

Axiom 3: NOT(Lion elem cage)

We observe the following invariant:

P1: C(L) v not(C(L))

where C(L) means: the value of "L" is in the cage.

Establishing C initially is trivially accomplished with the statement

;cage := {}

Note 0:

This is easily implemented by opening the door to the cage and shaking

out any lions that happen to be there initially.

(End of note 0.)

The obvious program structure is then:

;cage:={}

;do NOT (C(L)) ->

;"approach lion under invariance of P1"

;if P(L) ->

;"insert lion in cage"

[] not P(L) ->

;skip

;fi

;od

where P(L) means: the value of L is within arm's reach.

Note 1:

Axiom 2 esnures that the loop terminates.

(End of note 1.)

Exercise 0:

Refine the step "Approach lion under invariance of P1".

(End of exercise 0.)

Note 2:

The program is robust in the sense that it will lead to

abortion if the value of L is "lioness".

(End of note 2.)

Remark 0: This may be a new sense of the word "robust" for you.

(End of remark 0.)

Note 3:

From observation we can see that the above program leads to the

desired goal. It goes without saying that we therefore do not have to

run it.

(End of note 3.)

(End of approach.)