# Geeky! Robert Munafo’s RIES

http://www.mrob.com/pub/ries/index.html

ries (or RIES, an acronym for RILYBOT Inverse Equation Solver) takes any number and produces a list of equations that approximately solve to that number. Let’s try it with the answer to the question of live, the universe, and everything:

ries 42

Your target value: T = 42 http://www.mrob.com/ries

x+7 = 7^2 (exact match) {64}

l(l(x)) = 1/p+1 for X = T – 0.0239208 {76}

q(x+p) = 4+e for X = T – 0.00628193 {78}

x/5^2 = e/f for X = T – 0.000235975 {85}

q(x)-f = 9.C(1) for X = T + 0.00018202 {93}

5vx-1 = q(2/f) for X = T + 1.74556e-005{99}

1/(x+7)^2 = (1/7)^4 for X = T + 7.10543e-015{100}

e = base of natural logarithms, 2.71828… A v B = A root of B

f = phi, the golden ratio, (1+sqrt(5))/2 p = pi, 3.14159… C(x) = cos(x)

l = ln, natural logarithm or log base e q = square root

–LHS– –RHS– -Total-

dead-ends: 177399 243344 420743

expressions: 13265 19005 32270

distinct: 8066 8517 16583 Memory: 832K

Total equations tested: 68698122

RIES is presently featured on xkcd

It’s also fun to do with birthdays. Let’s try mine:

Your target value: T = 1981.0507 http://www.mrob.com/ries

xL5 = 2+e for X = T + 4.75828 {74}

3vx = 4.p for X = T + 3.35101 {71}

q(x/e) = 3^3 for X = T + 0.576753 {81}

x^S(2) = 9^p for X = T – 0.0873835 {86}

l(x)+6 = 5.e for X = T + 0.0525041 {87}

q(x)+q(p) = 2.e^p for X = T – 0.00572433 {95}

q(x/l(x)) = e^e+1 for X = T – 0.00236667 {100}