2^64
Roger K.W. Hui
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 [1],
us 2^64x eighteen quintillion, four hundred fortysix quadrillion, seven hundred fortyfour trillion, seventythree billion, seven hundred nine million, five hundred fiftyone 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 in [2].
Particles in the Universe
Is 2^64
larger than the number of particles in the universe?
Not even close [3, 4].
Avogadro Constant
The Avogadro constant[5] has value 6.022141e23 .
6.022141e23 % 2^64 32646.1
That the Avogardo constant is the number of atoms in twelve grams of carbon12 makes evident the enormity of the error of estimation in the previous section.
Age of the Universe
The age of the universe[6] 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 365.2425 24 60 60 6.31139e16
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 292.277 = (2^64) % 6.31e16
).
Supertanker Bytes
The largest tanker ever built, the Knock Nevis[7],
has a deadweight of 564,763 tonnes (tonne = 1000 kg)
and measures 1504 feet by 226 feet with a draft of 81 feet. A runofthemill 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).
We used to play a parlour game of wondering, “What’s the fastest way to send data across the
Atlantic?” Adapted for the current paper and for current technology, the question we may ask
is, “What is the fastest time to send 2^64
bytes across the Atlantic?” (A supertanker full of
disks/flash drives/DRAMs? An A380 full of same? Transmission by a 100 Gbps submarine cable?)
A rule of this game is that you must do the calculations in your head.
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[8]in every direction
 there is a tree every 5 feet
Therefore, the number of trees within your sight is:
NB. square feet within your sight o. *: 100 * 5280 8.75826e11 NB. # trees within your sight (*:5) %~ o. *: 100 * 5280 3.5033e10 NB. required # leaves on a tree (2^64) % (*:5) %~ o. *: 100 * 5280 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 semiannual 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 newborn 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 n2
rabbits that are alive at n2
months produce a pair, and these are added
to the existing population F n1
. Thus (F n) = (F n1) + (F n2)
.
F
is of course the Fibonacci sequence [9].
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 [10] 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[11] (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 [12] is a square matrix whose (i,j)th entry is %1+i+j
.
It is famously illconditioned 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.
References
 Hui, Roger K.W., Number in Words, Jwiki Essay, 20070712.
 Hui, Roger K.W., Ken Iverson Quotations and Anecdotes, 20050930. http://keiapl.org/anec/#rice

Bernecky, Robert, comp.lang.apl post, 19960331.
https://groups.google.com
search for groups or messages: “fewer than 2 power 60 particles in the universe” 
Hui, Roger K.W., comp.lang.apl post, 19960401.
(as above)  Avogadro constant http://en.wikipedia.org/wiki/Avogadro_constant
 Age of the universe http://en.wikipedia.org/wiki/Age_of_universe
 Knock Nevis http://en.wikipedia.org/wiki/Knock_Nevis
 You can see 100 miles http://en.wikipedia.org/wiki/Horizon
 Hui, Roger K.W., Fibonacci Sequence, Jwiki Essay, 20050926.
 Hui, Roger K.W., Partitions, Jwiki Essay, 20051118.
 Katana http://en.wikipedia.org/wiki/Katana
 Hui, Roger K.W., Hilbert Matrix, Jwiki Essay, 20050929.
An earlier version of this paper appeared as a an essay in the
Jwiki(
www.jsoftware.com/jwiki/Essays/2^64)
on 20071206.