# Essays/2^64

How big is` 2^64` ?

## Basics

2^64 1.84467e19 2^64x 18446744073709551616 'c0.0' 8!:2 ]2^64 18,446,744,073,709,551,616

And, using the verb` us `from Number in Words,

us 2^64x eighteen quintillion, four hundred forty-six quadrillion, seven hundred forty-four trillion, seventy-three billion, seven hundred nine million, five hundred fifty-one thousand, six hundred sixteen

## Grains on a Chessboard

One grain of rice is placed on the first square of an 8 by 8 chessboard,
two grains on the next square, four grains on the next, and so on,
doubling on each square. The total is of course` (2^64)-1 `grains.
How deep would that amount of rice cover the earth?
Answer.

## Particles in the Universe

Is` 2^64 `larger than the number of particles in the universe?
Not even close.

## Avogadro Constant

The Avogadro constant has value` 6.022141e23` .

6.022141e23 % 2^64 32646.1

That the Avogadro constant is the number of atoms in twelve grams of carbon-12 makes evident the enormity of the blunder in the previous section.

## Age of the Universe

The age of the universe is estimated to be about 14 billion years; its age in milliseconds is:

*/ 14e9 365.2425 24 60 60 1000 4.41797e20

## CPU Cycles

Assume the average modern PC is rated at 2 GHz. The number of CPU cycles in a year is therefore:

*/ 2e9 365 24 60 60 6.3072e16

The total of CPU cycles in a year for the computers found in a
residential neighborhood would exceed` 2^64` .` `(The
required number of computers is` 292.341 = (2^64) % 6.31e16` .)

## Supertanker Bytes

The largest tanker ever built,
the Knock Nevis,
has a deadweight of 564,763 tonnes (tonne = 1000 kg) and
measures 1504 feet by 226 feet with a draft of 81 feet.
A run-of-the-mill disk drive has a capacity of 200 GB,
and` 9e7 `drives would have a total capacity of` 2^64 `bytes.
Unless each drive exceeds 6 kg the tanker
would be able to carry them.

Might the tanker be constrained by volume?
Its volume exceeds` 27532224 = */ 1504 226 81 `cubic
feet which would readily accommodate` 9e7 `disk drives
(0.3 cubic foot per drive).

## Leaves on Trees

You stand on a mountain top in the North American Pacific Northwest
with trees in every direction.
Are there` 2^64 `leaves on the trees within your sight?
Estimate as follows:

- you can see 100 miles in every direction
- there is a tree every 5 feet

Therefore, the number of trees within your sight is:

o. *: 100 * 5280 NB. square feet within your sight 8.75826e11 (*:5) %~ o. *: 100 * 5280 NB. # trees within your sight 3.5033e10 (2^64) % (*:5) %~ o. *: 100 * 5280 NB. required # leaves on a tree 5.26553e8

Is it plausible for there to be` 5.27e8 `leaves on a tree?
There probably aren't that many leaves on an average deciduous tree.
However,
trees in the Pacific Northwest
are evergreen.` 5.27e8 `needles on an evergreen tree
seem possible (`22956.5 = %: 5.27e8` ;` `23 thousand branches each having 23 thousand needles).

## Compound Interest

How many years does it take to reach` 2^64 `dollars for $1 invested at
interest rate` r` ?
The equation for semi-annual compounding is:

(2^64) = (1+r%2)^2*y

Taking logarithms on both sides, we get` y = -: (1+r%2) ^. 2^64 `

] r=: 0.01 * 1+i.10 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 -: (1+r%2) ^. 2^64 4447.22 2229.14 1489.78 1120.09 898.273 750.393 644.761 565.536 503.914 454.614 r ,. >. -: (1+r%2) ^. 2^64 0.01 4448 0.02 2230 0.03 1490 0.04 1121 0.05 899 0.06 751 0.07 645 0.08 566 0.09 504 0.1 455

## Fibonacci's Rabbits

Fibonacci studied the population growth of (idealized) rabbits where:

- in the first month there is 1 newborn pair of rabbits
- a new-born pair becomes fertile from the 2nd month on
- each month every fertile pair begets a new pair
- rabbits never die

How many months are required for the number of rabbits to reach` 2^64` ?

Let` F n `be the number of pairs of rabbits after` n `months.
Only the` F n-2 `rabbits that are alive at` n-2 `months produce a pair,
and these are added to the existing population` F n-1 `.
Thus` (F n) = (F n-1) + (F n-2)` .
`F `is of course the Fibonacci sequence.

It can be shown that` (F n) = <. 0.5 + (%:5) %~ phi^n `where` phi `is the golden ratio` -:1+%:5 `.
The equation to be solved is:

(2^64) = 2 * (%:5) %~ phi^n

and the solution is:

phi=: -:1+%:5 phi ^. -: (%:5) * 2^64 92.4187

Less than 8 years.

## Factorial

The number of ways of arranging` n `distinct objects is` !n `.
What is the smallest` n `for which this exceeds` 2^64` ?

!^:_1 ]2^64 20.6671

## Partitions

A *partition* of` n `is a sorted list` x `of positive integers such that` n=+/x` .
For example, the following is the sorted list of all the partitions of 5:

┌─┬───┬───┬─────┬─────┬───────┬─────────┐ │5│4 1│3 2│3 1 1│2 2 1│2 1 1 1│1 1 1 1 1│ └─┴───┴───┴─────┴─────┴───────┴─────────┘

What is the smallest` n `for which the number of partitions of` n `exceeds` 2^64` ?

The verb` pnv `is from Partitions
where` pnv n `are the number of partitions for` i.1+n` .

p=: pnv 500 $ p 501 5 10 $ p 1 1 2 3 5 7 11 15 22 30 42 56 77 101 135 176 231 297 385 490 627 792 1002 1255 1575 1958 2436 3010 3718 4565 5604 6842 8349 10143 12310 14883 17977 21637 26015 31185 37338 44583 53174 63261 75175 89134 105558 124754 147273 173525 p (>i.1:) 2^64 417 ,. 416 417{p 17873792969689876004 18987964267331664557 2^64x 18446744073709551616

## Katana

To create a katana (samurai sword)
a billet of steel is heated and hammered, split and folded back upon itself many times.
If the number of foldings is greater than` 64 `then the
number of layers exceeds` 2^64` .

## E=m*c^2

With total conversion, how many kilograms of mass are required
to obtain` 2^64 `joules of energy?

(2^64) % *:3e8 204.964

## Square Inches

What is the radius in miles of a sphere whose surface area is` 2^64 `square inches?
The surface area of a sphere with radius` r `is` o.4**:r ` . Thus:

(*/ 12 5280) %~ %: (2^64) % o.4 19122.3

Such a sphere is a little larger than Uranus.

## Cubic Inches

What is the radius in miles of a sphere whose volume is` 2^64 `cubic inches?
The volume of a sphere with radius` r `is` o.(4%3)*r^3 ` . Thus:

(*/ 12 5280) %~ 3 %: (2^64) % o.4%3 25.8699

## Hilbert Matrix

The Hilbert matrix is a square matrix
whose` `(`i,j`)-th entry is` %1+i+j` .` `It is famously
ill-conditioned with a very small magnitude determinant.

H=: % @: >: @: (+/~) @: i. H 5x 1 1r2 1r3 1r4 1r5 1r2 1r3 1r4 1r5 1r6 1r3 1r4 1r5 1r6 1r7 1r4 1r5 1r6 1r7 1r8 1r5 1r6 1r7 1r8 1r9 det=: -/ .* det H 5x 1r266716800000 %. H 5x 25 _300 1050 _1400 630 _300 4800 _18900 26880 _12600 1050 _18900 79380 _117600 56700 _1400 26880 _117600 179200 _88200 630 _12600 56700 _88200 44100

The inverse Hilbert matrix has all integer entries, whose (integer) determinant is very large.

>./ | , %. H 15x 114708987924290760000 >./ | , %. H 14x 3521767173114190000 % det H 7x 2067909047925770649600000 % det H 6x 186313420339200000 perm=: +/ .* perm %. H 5x 4855173934730716800000 perm %. H 4x 5314794912000

The smallest inverse Hilbert matrix with
an entry that exceeds` 2^64 `in absolute value
is the one of order` 15` ;` `with
a determinant that exceeds` 2^64` ,` `order` 7` ;` `with
a permanent that exceeds` 2^64` ,` `order` 5` .

## Making $$$

In U.S. dollars the units in common circulation are:

- bills: 100 50 20 10 5 1
- coins: 0.25 0.10 0.05 0.01

A dollar can be "made" in a number of ways:

1.00 0.25 0.10 0.05 0.01 0 0 0 0 100 0 0 0 1 95 0 0 0 2 90 ... 0 3 2 1 0 0 4 0 0 0 1 0 0 0 0

In fact, a dollar can be made in 243 ways.
What is the smallest multiple of $100 that can
be made in greater than` 2^64 `ways?

h=: 4 : 0 m=. # s=. +/\ y if. 2.5=x do. (m$5{.1)#m($,)+/\_2]\s else. (m$x{.1)#s end. ) chm=: 3 : '+/ 2 h 2.5 h 2 h 5 h 4 h 2.5 h (*y)$~1+20*y' " 0

If` n `is a multiple of` 100 `then` chm n `is the number of
ways of making` n `dollars.

chm 100*>:i.3 5 4.88209e10 4.35246e12 7.62895e13 6.46316e14 3.58401e15 1.50147e16 5.149e16 1.51912e17 3.98556e17 9.51655e17 2.10326e18 4.35756e18 8.54636e18 1.59902e19 2.87178e19 chm 1400 1500 1.59902e19 2.87178e19

$1500 can be made in` 2.87e19 `ways. The exact number is:

chm 1500x 28717791430084742056

Suppose the more rarely circulated $2 bill and 50 cent coin are included. Then:

chn =: 3 : '+/ 2 h 2.5 h 2 h 2.5 h 2 h 2 h 2.5 h (*y)$~1+20*y' " 0 chn 100*>:i.3 5 9.82355e12 2.78e15 9.69549e16 1.34924e18 1.10638e19 6.40915e19 2.90001e20 1.09038e21 3.54917e21 1.02915e22 2.71434e22 6.61402e22 1.50698e23 3.24114e23 6.63033e23 chn 500 600 1.10638e19 6.40915e19 chn 600x 64091464225604008941

$600 can be made in` 6.41e19 `ways,
and is the smallest multiple of $100 than can
be made in greater than` 2^64 `ways.

### See also

Contributed by Roger Hui.