When i try this sym('x')^y i get an error message

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Joe Bennett 2020-12-11

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评论： Joe Bennett 2020-12-11

a = sym(18008617784390347685963)^60322355516214665580

I get an error message, can anyone help please.

Cheers

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Answer 1

Vladimir Sovkov 2020-12-11

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You cannot compute this directly because the integers are larger than matlab is able to treat.

You can try

b = sym(log10(18008617784390347685963)*60322355516214665580)
 
b =
 
1342502999708137684992

so that the value you are interested in is

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Vladimir Sovkov 2020-12-11

编辑：Vladimir Sovkov 2020-12-11

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or, even better

nd=64; % number of decimal digits you want to get your result with
b = vpa(sym(log10(sym(18008617784390347685963))*60322355516214665580),nd)
% b = 1342502999708137528766.63000133154837392990340259044889069097638

a=10^mod(b,1)

\times 10^ (floor(b))

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Answer 2

Daniel Pollard 2020-12-11

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编辑：Daniel Pollard 2020-12-11

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I haven't used the symbolic toolbox before, but when I type 18008617784390347685963^60322355516214665580 into my Matlab commandline it returns Inf. Computers can't handle very big numbers, and that number will be one with over 1e21 digits. You haven't told us the error message (which you should, as a rule) but this seems most likely. Google can act as a calculator and that returns 'undefined'.

What on earth do you need to store a number that big for anyway?

Edit A correction - I hilariously underestimated how many digits the number would have. It's over 1e21. I tried

randi([1 9], 1, 1e11)

and Matlab tells me that array would require 750GB of RAM to create. This has made me more curious as to what this number is you're trying to calculate.

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Joe Bennett 2020-12-11

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so im trying to use matlab to do the digital signature algorithm...

p = 30167674936870980426367;
q = 17456345243;
g = 18008617784390347685963;
y = 6172647251731232412543;
H = 15296664068;
r = 16772231458;
s = 3953283568;
% % let u_1 = u;
% % let u_2 = v;
% % H = mod(s*u, q)
% % r = mod(s*v, q) 
syms u k integer               
eqn = s*u - q*k == H;      
[u, k] = solve(eqn,[u k]);  
u
syms v l integer               
eqn = s*v - q*l == r;      
[v, l] = solve(eqn,[v l]);  
v
a = sym(g)^u;
b = sym(y)^v;
remainder = powermod(10, a+b, p)
mod(remainder, q) % should equal r
This is my code, all the values at the top were given. For the end of this algorithm i need to find the remainder of (g^u)*(y^v) | p. 