Trying to compare two array elements and getting error

I got 2 arrays in my program, the first one is numbers and the second one is not, while trying to compare them, I am getting this error :

./P5.4.sh: line 48: [: /: integer expression expected
./P5.4.sh: line 48: [: *: integer expression expected
./P5.4.sh: line 36: [[: -: syntax error: operand expected (error token is "-")

this is line 36 :

        if [[ (( "${formula[i-1]}" -ge "0" &&  "${formula[i-1]}" -le "9$
        (( "${formula[i+1]}" -ge "0" && "${formula[i+1]}" -le "9" )) ]]

and this is line 48 :

if [ "${formula[i+1]}" -ge "0" ] && [ "${formula[i+1]}" -le "9" ]

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