I input:

solve({My(x, -(1/2)*b) = 0, My(x, (1/2)*b) = 0, w(x, -(1/2)*b) = 0, w(x, (1/2)*b) = 0}, {Am, Bm, Cm, Dm});

and recieved: 

Error, (in My) invalid input: diff received -1, which is not valid for its 2nd argument

My is 

My := proc (x, y) options operator, arrow; -((1/2)*I)*Bm*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x-y)/a)/((exp(Pi*(I*x+y)/a)-1)*(-1+exp(Pi*(I*x-y)/a))^2*a^2)+I*Dm*y*(exp((2*I)*Pi*x/a)-1)*Pi^2*(exp(Pi*(I*x-y)/a))^2/((exp(Pi*(I*x+y)/a)-1)*(-1+exp(Pi*(I*x-y)/a))^3*a^2)+I*Dm*y*(exp((2*I)*Pi*x/a)-1)*Pi^2*(exp(Pi*(I*x+y)/a))^2/((exp(Pi*(I*x+y)/a)-1)^3*(-1+exp(Pi*(I*x-y)/a))*a^2)-I*Bm*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x+y)/a)*exp(Pi*(I*x-y)/a)/((exp(Pi*(I*x+y)/a)-1)^2*(-1+exp(Pi*(I*x-y)/a))^2*a^2)+I*Bm*(exp((2*I)*Pi*x/a)-1)*Pi^2*(exp(Pi*(I*x+y)/a))^2/((exp(Pi*(I*x+y)/a)-1)^3*(-1+exp(Pi*(I*x-y)/a))*a^2)-I*Dm*y*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x+y)/a)*exp(Pi*(I*x-y)/a)/((exp(Pi*(I*x+y)/a)-1)^2*(-1+exp(Pi*(I*x-y)/a))^2*a^2)-((1/2)*I)*Bm*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x+y)/a)/((exp(Pi*(I*x+y)/a)-1)^2*(-1+exp(Pi*(I*x-y)/a))*a^2)-((1/2)*I)*Dm*y*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x+y)/a)/((exp(Pi*(I*x+y)/a)-1)^2*(-1+exp(Pi*(I*x-y)/a))*a^2)-I*Dm*(exp((2*I)*Pi*x/a)-1)*Pi*exp(Pi*(I*x+y)/a)/((exp(Pi*(I*x+y)/a)-1)^2*(-1+exp(Pi*(I*x-y)/a))*a)+I*Bm*(exp((2*I)*Pi*x/a)-1)*Pi^2*(exp(Pi*(I*x-y)/a))^2/((exp(Pi*(I*x+y)/a)-1)*(-1+exp(Pi*(I*x-y)/a))^3*a^2)+I*Dm*(exp((2*I)*Pi*x/a)-1)*Pi*exp(Pi*(I*x-y)/a)/((exp(Pi*(I*x+y)/a)-1)*(-1+exp(Pi*(I*x-y)/a))^2*a)-((1/2)*I)*Dm*y*(exp((2*I)*Pi*x/a)-1)*Pi^2*exp(Pi*(I*x-y)/a)/((exp(Pi*(I*x+y)/a)-1)*(-1+exp(Pi*(I*x-y)/a))^2*a^2)+sum(-4*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(diff(d(y), y), y))/(Pi(y)^7*m(y)^7*d(y)^2)+32*(diff(po(y), y))*a(y)^3*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(a(y), y))/(Pi(y)^7*m(y)^7*d(y))-56*(diff(po(y), y))*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(Pi(y), y))/(Pi(y)^8*m(y)^7*d(y))-56*(diff(po(y), y))*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(m(y), y))/(Pi(y)^7*m(y)^8*d(y))-8*(diff(po(y), y))*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(d(y), y))/(Pi(y)^7*m(y)^7*d(y)^2)+32*po(y)*a(y)^3*(2*Pi(y)*m(y)^2*y(y)^2*(diff(Pi(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(m(y), y))+2*Pi(y)^2*m(y)^2*y(y)*(diff(y(y), y))+8*a(y)*(diff(a(y), y)))*sin(m*Pi*x/a)*(diff(a(y), y))/(Pi(y)^7*m(y)^7*d(y))+16*po(y)*a(y)^3*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(diff(a(y), y), y))/(Pi(y)^7*m(y)^7*d(y))-56*po(y)*a(y)^4*(2*Pi(y)*m(y)^2*y(y)^2*(diff(Pi(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(m(y), y))+2*Pi(y)^2*m(y)^2*y(y)*(diff(y(y), y))+8*a(y)*(diff(a(y), y)))*sin(m*Pi*x/a)*(diff(Pi(y), y))/(Pi(y)^8*m(y)^7*d(y))-56*po(y)*a(y)^4*(2*Pi(y)*m(y)^2*y(y)^2*(diff(Pi(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(m(y), y))+2*Pi(y)^2*m(y)^2*y(y)*(diff(y(y), y))+8*a(y)*(diff(a(y), y)))*sin(m*Pi*x/a)*(diff(m(y), y))/(Pi(y)^7*m(y)^8*d(y))-8*po(y)*a(y)^4*(2*Pi(y)*m(y)^2*y(y)^2*(diff(Pi(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(m(y), y))+2*Pi(y)^2*m(y)^2*y(y)*(diff(y(y), y))+8*a(y)*(diff(a(y), y)))*sin(m*Pi*x/a)*(diff(d(y), y))/(Pi(y)^7*m(y)^7*d(y)^2)-224*po(y)*a(y)^3*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(a(y), y))*(diff(Pi(y), y))/(Pi(y)^8*m(y)^7*d(y))-224*po(y)*a(y)^3*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(a(y), y))*(diff(m(y), y))/(Pi(y)^7*m(y)^8*d(y))-32*po(y)*a(y)^3*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(a(y), y))*(diff(d(y), y))/(Pi(y)^7*m(y)^7*d(y)^2)+392*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(Pi(y), y))*(diff(m(y), y))/(Pi(y)^8*m(y)^8*d(y))+56*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(Pi(y), y))*(diff(d(y), y))/(Pi(y)^8*m(y)^7*d(y)^2)+56*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(m(y), y))*(diff(d(y), y))/(Pi(y)^7*m(y)^8*d(y)^2)+4*(diff(diff(po(y), y), y))*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)/(Pi(y)^7*m(y)^7*d(y))+8*(diff(po(y), y))*a(y)^4*(2*Pi(y)*m(y)^2*y(y)^2*(diff(Pi(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(m(y), y))+2*Pi(y)^2*m(y)^2*y(y)*(diff(y(y), y))+8*a(y)*(diff(a(y), y)))*sin(m*Pi*x/a)/(Pi(y)^7*m(y)^7*d(y))+4*po(y)*a(y)^4*(2*(diff(Pi(y), y))^2*m(y)^2*y(y)^2+8*Pi(y)*m(y)*y(y)^2*(diff(Pi(y), y))*(diff(m(y), y))+8*Pi(y)*m(y)^2*y(y)*(diff(Pi(y), y))*(diff(y(y), y))+2*Pi(y)*m(y)^2*y(y)^2*(diff(diff(Pi(y), y), y))+2*Pi(y)^2*(diff(m(y), y))^2*y(y)^2+8*Pi(y)^2*m(y)*y(y)*(diff(m(y), y))*(diff(y(y), y))+2*Pi(y)^2*m(y)*y(y)^2*(diff(diff(m(y), y), y))+2*Pi(y)^2*m(y)^2*(diff(y(y), y))^2+2*Pi(y)^2*m(y)^2*y(y)*(diff(diff(y(y), y), y))+8*(diff(a(y), y))^2+8*a(y)*(diff(diff(a(y), y), y)))*sin(m*Pi*x/a)/(Pi(y)^7*m(y)^7*d(y))-28*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(diff(m(y), y), y))/(Pi(y)^7*m(y)^8*d(y))+48*po(y)*a(y)^2*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(a(y), y))^2/(Pi(y)^7*m(y)^7*d(y))+224*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(Pi(y), y))^2/(Pi(y)^9*m(y)^7*d(y))+224*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(m(y), y))^2/(Pi(y)^7*m(y)^9*d(y))+8*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(d(y), y))^2/(Pi(y)^7*m(y)^7*d(y)^3)-28*po(y)*a(y)^4*(Pi(y)^2*m(y)^2*y(y)^2+4*a(y)^2)*sin(m*Pi*x/a)*(diff(diff(Pi(y), y), y))/(Pi(y)^8*m(y)^7*d(y)), m = 1 .. infinity) end proc;

[Lengthy, poorly formatted, and very-difficult-to-read plaintext prettyprint of the above procedure removed by a moderator.--Carl Love]