Multiply with restricted operations



There's a 500 rep unofficial bounty for beating the current best answer.


Your goal is to multiply two numbers using only a very limited set of arithmetic operations and variable assignment.

  1. Addition x,y -> x+y
  2. Reciprocal x -> 1/x (not division x,y -> x/y)
  3. Negation x -> -x (not subtraction x,y -> x-y, though you can do it as two operations x + (-y))
  4. The constant 1 (no other constants allowed, except as produced by operations from 1)
  5. Variable assignment [variable] = [expression]

Scoring: The values start in variables a and b. Your goal is to save their product a*b into the variable c using as few operations as possible. Each operation and assignment +, -, /, = costs a point (equivalently, each use of (1), (2), (3), or (4)). Constants 1 are free. The fewest-point solution wins. Tiebreak is earliest post.

Allowance: Your expression has to be arithmetically correct for "random" reals a and b. It can fail on a measure-zero subset of R2, i.e. a set that has no area if plotted in the a-b Cartesian plane. (This is likely to be needed due to reciprocals of expressions that might be 0 like 1/a.)


This is an . No other operations may be used. In particular, this means no functions, conditionals, loops, or non-numerical data types. Here's a grammar for the allowed operations (possibilities are separated by |). A program is a sequence of <statement>s, where a <statement> is given as following.

<statement>: <variable> = <expr>
<variable>: a | b | c | [string of letters of your choice]
<expr>: <arith_expr> | <variable> | <constant>
<arith_expr>: <addition_expr> | <reciprocal_expr> | <negation_expr> 
<addition_expr>: <expr> + <expr>
<reciprocal_expr>: 1/(<expr>)
<negation_expr>: -<expr>
<constant>: 1

You don't actually have to post code in this exact grammar, as long as it's clear what you're doing and your operation count is right. For example, you can write a-b for a+(-b) and count it as two operations, or define macros for brevity.

(There was a previous question Multiply without Multiply, but it allowed a much looser set of operations.)


Posted 2014-09-01T20:03:44.447

Reputation: 115 687

@xnor Currently, I have the answer which works best without assignment allowed (assignment used only once, and will only double it (approximately)). Does that count for the optimal result without assignment, or does it need to be proven to be the optimal result? – proud haskeller – 2015-05-02T17:58:13.477

@proudhaskeller No, I'm looking for a proof of optimality. (Sorry, I had missed your comment in my feed.) – xnor – 2015-05-07T20:30:01.683

4Is this even possible? – Ypnypn – 2014-09-01T20:19:44.460

1@Ypnypn Yes, and I've written an example to make sure. – xnor – 2014-09-01T20:24:15.563

2This feels like a challenge where an optimal solution is likely to be found (once any solution has been found). So what's the tie breaker in that case? – Martin Ender – 2014-09-01T20:27:27.033

1@MartinBüttner Tiebreak is earliest posting in that case. I think there's a good amount of room for optimizations, so I don't think it will just be a race to find one that works and write it cleanly. At least, that's what I found in trying it; maybe someone will find a clearly minimal solution. – xnor – 2014-09-01T20:30:39.550

2Ok since not everyone thought my anwer was as funny as I did, I deleted it and comment here: The rule about the measure zero set is not very wisely chosen since rational numbers are a measure zero set regarding the lebesgue measure, I'd suggest using a certain percentage instead. (Or another kind) But I totally like the idea of this challenge! – flawr – 2014-09-01T21:43:54.903

1@flawr I was imagining the fake language to express arithmetic formulas for real numbers, not act on bits or any sort of finite representation, so I think the measure requirement over the real plane (a,b) ∈ R^2 works fine. But if you actually think it's unclear, I'll edit it. – xnor – 2014-09-01T21:45:41.183

1Of course it is clear what you mean but I think it does not help using precisely defined technical terminology for something that does not meet the definition. Sorry for being a stickler! In my opinion you can also write I know what you mean or (most as in common sense). – flawr – 2014-09-01T21:47:41.510

@flawr I was typing up the same stuff when I saw your comments. Exactly, the set of IEEE floating point numbers is measure zero, and so is all pairs of them. And so is the set of all numbers representable on any machine. – Eric Tressler – 2014-09-01T21:51:08.407

1OK, thanks for the feedback, I guess I should have realize that coders would interpret numbers as stored in bits. I edited it to try to make clear I'm requiring arithmetically correct results for all but a measure-zero set of pairs of real numbers. – xnor – 2014-09-01T21:53:17.723

@xnor How much operations you got when you solved this question? – proud haskeller – 2014-09-04T05:17:33.703

@proudhaskeller 33 :-P It was basically a worse version of algorithmshark's answer. – xnor – 2014-09-04T05:23:17.213

1I don't know that there's a better choice for an algorithmic search other than brute force... And if the actual best solution is, say, 22 operations, that's not really feasible for a brute force search. – rationalis – 2014-09-04T23:57:58.867

I will give a bounty of 300 rep to anyone who posts an optimal solution with proof. A brute force search can be a proof if you explain how it checks everything needed. An optimal result restricted not to use variable assignment will be given 150 rep. Other partial bounties may be given for partial results. – xnor – 2014-10-27T17:45:08.997

@xnor What do you mean by optimal here ? less than 23 operations ? – Optimizer – 2014-10-27T17:50:58.243

@Optimizer No, the best theoretically possible. It might be 23, it might be less. A solution of 22 operations with no proof of optimality would not suffice. (Clarified in the post, thanks.) – xnor – 2014-10-27T17:52:21.707

You mean 22 operations (instead of chars), right ? Also, if someone posts a 24 operations solution, will that count ? – Optimizer – 2014-10-27T17:56:52.140

@Optimizer Yes, operations, thanks. Not sure what you mean about a 24 op solution -- it can't be optimal because there's a 23. If you mean for the no-assignment half-prize, yes, I'd count an optimal solution for that that's more than 23 ops. – xnor – 2014-10-27T17:58:51.777

I see. So the existing 23 operations won't get 300 if no one else posts, right ? Also, are you really sure an answer less than 23 exists ? – Optimizer – 2014-10-27T17:59:53.983



22 operations

itx = 1/(1+a+b)     #4
nx = -1/(itx+itx)   #4
c = -( 1/(itx + itx + 1/(1+nx)) + 1/(1/(a+nx) + 1/(b+nx)) ) #14

Try it online!

The ops are 10 additions, 7 inverses, 2 negations, and 3 assignments.

So, how did I get this? I started with the promising-looking template of the sum of two double-decker fractions, a motif that had appeared in many previous attempts.

c = 1/(1/x + 1/y) + 1/(1/z + 1/w)

When we restrict the sum to x+y+z+w=0, a beautiful cancellations occur, giving:

c = (x+z)*(y+z)/(x+y),

which contains a product. (It's often easier to get t*u/v rather than t*u because the first has degree 1.)

There's a more symmetric way to think about this expression. With the restriction x+y+z+w=0, their values are specified by three parameters p,q,r of their pairwise sums.

 p = x+y
-p = z+w
 q = x+z
-q = y+w
 r = x+w
-r = y+z

and we have c=-q*r/p. The sum p is distinguished as being in the denominator by corresponding to the pairs (x,y) and (z,w) of variables that are in the same fraction.

This is a nice expression for c in p,q,r, but the double-decker fraction is in x,y,z,w so we must express the former in terms of the latter:

x = ( p + q + r)/2
y = ( p - q - r)/2
z = (-p + q - r)/2
w = (-p - q + r)/2

Now, we want to choose p,q,r so that c=-q*r/p equals a*b. One choice is:

p = -4
q = 2*a
r = 2*b

Then, the doubled values for q and r are conveniently halved in:

x = -2 + a + b
y = -2 - a - b
z =  2 + a - b
w =  2 - a + b

Saving 2 as a variable t and plugging these into the equation for c gives a 24-op solution.

#24 ops
t = 1+1   #2
c = 1/(1/(-t+a+b) + 1/-(t+a+b))  +  1/(1/(-b+t+a) + 1/(-a+b+t)) #1, 10, 1, 10

There's 12 additions, 6 inverses, 4 negations, and 2 assignments.

A lot of ops are spent expressing x,y,z,w in terms of 1,a,b. To save ops, instead express x in p,q,r (and thus a,b,1) and then write y,z,w in terms of x.

y = -x + p
z = -x + q
w = -x + r


p = 1
q = a
r = b

and expressing c with a negation as c=-q*r/p, we get

x = (1+a+b)/2
y = -x + 1
z = -x + a
w = -x + b

Unfortunately, halving in x is costly. It needs to be done by inverting, adding the result to itself, and inverting again. We also negate to produce nx for -x, since that's what y,z,w use. This gives us the 23-op solution:

#23 ops
itx = 1/(1+a+b)     #4
nx = -1/(itx+itx)   #4
c = -( 1/(1/(-nx) + 1/(1+nx))  +  1/(1/(a+nx) + 1/(b+nx)) ) #15

itx is 1/(2*x) and nx is -x. A final optimization of expressing 1/x as itx+itx instead of the templated1/(-nx) cuts a character and brings the solution down to 22 ops.


Posted 2014-09-01T20:03:44.447

Reputation: 115 687

There's an easy optimisation to 21 operations. itx + itx occurs twice, but itx doesn't occur in any other context. Define instead ix = (1+1)/(1+a+b) and you replace two additions with one. – Peter Taylor – 2017-01-27T10:51:07.603

And by extracting m = -1 it's possible to get 20: nx = (1+a+b)/(m+m); c = m/(m/nx + 1/(1+nx)) + m/(1/(a+nx) + 1/(b+nx)) – Peter Taylor – 2017-01-27T10:57:55.123

3Ah, both of those optimisations fail because the supported operation is reciprocal rather than division. – Peter Taylor – 2017-01-27T11:01:27.800

If a and b are only one apart, then either a + nx = 0 or b + nx = 0, causing your solution to divide by zero. – MooseOnTheRocks – 2019-03-17T02:14:00.020

1@MooseOnTheRocks That's fine, see the "allowance" in the challenge that the code can fail on a measure-zero subset. I think the challenge is impossible otherwise. – xnor – 2019-03-17T02:18:59.663


23 operations

z = 1/(1/(1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1))-(1/a+1/b))
res = z+z

proof by explosion:

z = 1/(1/(1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1))-(1/a+1/b))
             1/(a+1)+1/(b+1)                            == (a+b+2) / (ab+a+b+1)
          1/(1/(a+1)+1/(b+1))                           == (ab+a+b+1) / (a+b+2)
          1/(1/(a+1)+1/(b+1))-1                         == (ab - 1) / (a+b+2)
          1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1)             == ab / (a+b+2)
       1/(1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1))            == (a+b+2) / ab
                                              1/a+1/b   == (a+b) / ab
       1/(1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1))-(1/a+1/b)  == 2 / ab
    1/(1/(1/(1/(a+1)+1/(b+1))-1+1/(a+b+1+1))-(1/a+1/b)) == ab / 2

z = ab / 2 and therefore z+z = ab

I abused wolfram alpha to get this beautiful image (wolfram alpha tried to get me to subscribe to pro to save it, but then ctrl-c ctrl-v ;-)):

score (with added + on subtraction):

z = ////++/++-+/++++-/+/
res = +

proud haskeller

Posted 2014-09-01T20:03:44.447

Reputation: 5 866

Not to be nit-picky, but shouldn't ...(b+1))-1+1... and ...1))-(1/a+... be ...(b+1))+-1+1... and ...1))+-(1/a+... respectively? – tfitzger – 2015-05-02T17:51:00.870

@tfitzger I think it's easier that way. The question does say that it doesn't matter. Note I do count the score correctly (Every minus is a two) – proud haskeller – 2015-05-02T17:53:38.380

Wolfram Alpha has a 7-day free trial, fyi. – ghosts_in_the_code – 2015-11-27T18:05:29.230

@ghosts_in_the_code I had already used that at the time – proud haskeller – 2015-11-27T18:34:23.793

@proudhaskeller You can always create dummy accounts and keep using the trial. – ghosts_in_the_code – 2015-11-28T03:22:17.650

"Proof by explosion" has got to be the coolest-sounding method of proof out there. – mbomb007 – 2017-04-06T16:14:41.193

Congrats on the shortest solution! – xnor – 2014-09-11T04:04:45.680

@xnor thanks for giving me my first accepted answer and my first bounty! – proud haskeller – 2014-09-11T05:33:31.840


29 operations

Does not work for the set { (a,b) ∈ R2 | a+b = 0 or a+b = -1 or a-b = 0 or a-b = -1 }. That's probably measure zero?

sum = a+b
nb = -b
diff = a+nb
rfc = 1/(1/(1/sum + -1/(sum+1)) + -1/(1/diff + -1/(diff+1)) + nb + nb)  # rfc = 1/4c
c = 1/(rfc + rfc + rfc + rfc)

# sum  is  2: =+
# nb   is  2: =-
# diff is  2: =+
# rfc  is 18: =///+-/++-//+-/+++
# c    is  5: =/+++
# total = 29 operations

The structure of rfc (Reciprocal-Four-C) is more evident if we define a macro:

s(x) = 1/(1/x + -1/(x+1))              # //+-/+ (no = in count, macros don't exist)
rfc = 1/(s(sum) + - s(diff) + nb + nb) # =/s+-s++ (6+2*s = 18)

Let's do the math:

  • s(x), mathematically, is 1/(1/x - 1/(x+1)) which is after a bit of algebra is x*(x+1) or x*x + x.
  • When you sub everything into rfc, it's really 1/((a+b)*(a+b) + a + b - (a-b)*(a-b) - a + b + (-b) + (-b)) which is just 1/((a+b)^2 - (a-b)^2).
  • After difference of squares, or just plain expansion, you get that rfc is 1/(4*a*b).
  • Finally, c is the reciprocal of 4 times rfc, so 1/(4/(4*a*b)) becomes a*b.


Posted 2014-09-01T20:03:44.447

Reputation: 8 144

2+1, I was in the middle of finishing up this identical calculation – Eric Tressler – 2014-09-01T21:19:14.630

1That's definitely measure zero; it's a union of lines. – xnor – 2014-09-01T21:54:15.973

Not gonna make a comment about union of lines... @algorithmshark Can you tell us more how you did come up with this identity? How did you aproach the problem? – flawr – 2014-09-02T12:31:29.577

1@flawr I recalled that the properties of s(x) fit the requirements of the question, from calculus, so that meant I had a square function. After some faffing about, I found I could get an a*b term with the difference of squares trick. Once I had that, it was a matter of trying out which assignments saved operations. – algorithmshark – 2014-09-02T15:22:21.720

Since you use -1 three times in rfc, couldn't you golf a character out by assigning it to a variable? – isaacg – 2014-09-02T18:23:59.283

@isaacg notice he didn't use (-1)/(..), as division is not a legal operation, but -(1/(...)) – proud haskeller – 2014-09-02T21:53:41.840


27 operations

tmp = 1/(1/(1+(-1/(1/(1+(-a))+1/(1+b))))+1/(1/(1/b+(-1/a))+1/(a+(-b))))
res = tmp+tmp+(-1)

# tmp is 23: =//+-//+-+/++///+-/+/+-
# res is 4: =++-

There is no theory behind this. I just tried to get (const1+a*b)/const2 and started with (1/(1-a)+1/(1+b)) and (-1/a+1/b).


Posted 2014-09-01T20:03:44.447


Your tmp is actually 23, making your score 27. Nice find, though. – algorithmshark – 2014-09-02T15:29:16.740