Thank you!

Thank you!

Thank you, working also

sum('p'(k), k=1..1)

But if I changed the constant -1 to any other fixed value, for example

p := proc (k) sum((-2)^n*n/(factorial(k-n)*factorial(n)), n = 1 .. k) end proc;

or

p := proc (k) sum((-4)^n*n/(factorial(k-n)*factorial(n)), n = 1 .. k) end proc;

then quotes are not necessary and output is right! Quotes are necessary only for value -1. Very mysterious property.

Thank you, working also

sum('p'(k), k=1..1)

But if I changed the constant -1 to any other fixed value, for example

p := proc (k) sum((-2)^n*n/(factorial(k-n)*factorial(n)), n = 1 .. k) end proc;

or

p := proc (k) sum((-4)^n*n/(factorial(k-n)*factorial(n)), n = 1 .. k) end proc;

then quotes are not necessary and output is right! Quotes are necessary only for value -1. Very mysterious property.