ODE error message "Error using exist The first input to exist must be a string scalar or character vector"

this is my code

syms no(k1,k2,k3,k_1,k_2,k_3,o2,n2,o,h2o,oh,h,t)
n=(k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh);
eqn1=k1*o*n2-k_1*no*n+k2*n*o2-k_2*no*o+k3*n*oh-k_3*no*h;
fin = diff(no,t)==eqn1;
tspan = [0 10];
i=[0 0];
ra=ode45(fin,tspan,i)

[darova]

• you are using symbolic variables. ode45 needs function or function handle
• your no function depends on several variables (a lot), ode45 works only with one independent variable (ordinary differential equation)

Can you attach original equations?

[Abdelrahman Eldaly]

here no function only in t i want no(t). Unable to find explicit solution appears

T=2146;
a=(101.325/(8.314*T))*10^-3;
o2=16.8*a; n2=717*a; o=0.245*a; h2o=171*a; oh=2.75*a; h=0.134*a;
k1=1.8*10^14*exp(-38370/T);
k_1=3.8*10^13*exp(-425/T);
k2=1.8*10^10*T*exp(-4680/T);
k_2=3.8*10^9*T*exp(-28280/T);
k3=7.1*10^13*exp(-450/T);
k_3=1.7*10^14*exp(-24560/T);
syms no(t)
fin = diff(no,t)==k1*o*n2-k_1*no*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))+k2*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*o2-k_2*no*o+k3*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*oh-k_3*no*h;
cond= no(0)==0;
no(t)=dsolve(fin,cond)

Answer 1

[darova]

Try this way: prepare function handle for ode45 solver

fin = @(t,no) k1*o*n2-k_1*no*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))+k2*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*o2-k_2*no*o+k3*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*oh-k_3*no*h;
[t,no] = ode45(fin,[0 1e-7],0);
plot(t,no)

Answer 2

[Abdelrahman Eldaly]

it should appear like this. where is the error

T=2146;
a=(101.325/(8.314*T))*10^-3;
o2=16.8*a; n2=717*a; o=0.245*a; h2o=171*a; oh=2.75*a; h=0.134*a;
k1=1.8*10^14*exp(-38370/T);
k_1=3.8*10^13*exp(-425/T);
k2=1.8*10^10*T*exp(-4680/T);
k_2=3.8*10^9*T*exp(-28280/T);
k3=7.1*10^13*exp(-450/T);
k_3=1.7*10^14*exp(-24560/T);
fin = @(t,no) k1*o*n2-k_1*no*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))+k2*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*o2-k_2*no*o+k3*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*oh-k_3*no*h;
tim=0:0.1:2;
[t,no] = ode45(fin,tim,0);
plot(t,no)

[darova]

Can you show the original equation?

[Abdelrahman Eldaly]

[darova]

The only idea i have is to separate this long expression into two parts. Because it's hard to check if it's correct or not

% fin = @(t,no) k1*o*n2-k_1*no*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))+k2*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*o2-k_2*no*o+k3*((k1*o*n2+k_2*no*o+k_3*no*h)/(k_1*no+k2*o2+k3*oh))*oh-k_3*no*h;
N = @(no) (...)/(...);
fin = @(t,no) k1*o*n2 - k_1*no*N(no) + ...