a. 26 = 4 x (5 + 3 / 2)

b. 26 = 5 x (3/2 - 1 + 4)

c. 26 = (51+4-3)/2

d. 26 = 5 x (2+4) - 3 - 1

calc := function(pairs, ops)
local l, i, p, op, x, y, r, ind;

l := [1,2,3,4,5];

for i in [1..4] do
p := pairs[i];
op := ops[i];
x := l[p‏¹];
y := l[p‏²];

if op=1 then r := x+y; fi;
if op=2 then r := x-y; fi;
if op=3 then r := x*y; fi;
if op=4 then
if y = 0 then r := 999999; else r := x/y; fi;
fi;

l[p‏¹]:=r;
ind := [1..6-i]; RemoveSet(ind, p‏²);
l := l{ind};
od;

return l‏¹;

end;

allpairs := Cartesian(Arrangements([1..5],2), Arrangements([1..4],2),
Arrangements([1..3],2), Arrangements([1..2],2));
allops := PermutationsList([1..4]);

for pairs in allpairs do for ops in allops do
c := calc(pairs, ops);
if c = 26 then Print(pairs, " ",ops, "\\n"); fi;
od; od;

\\sigma_{n=1}^\\infty n/(2^n)

\\sigma_{n=1}^\\infty \\sigma_{i=n}^\\infty 1/(2^i)

\\sigma_{n=1}^\\infty 1/(2^(n-1))

(2n+1)^2

2n(2n+1)

(n-1)2^2