adjacent_elements_product() {
    local -n _arr="$1"
    local _max=-9223372036854775808
    local _i _c=${#_arr[@]}
    for ((_i = 0; _i < _c - 1; _i++)); do
        local _p=$(( _arr[_i] * _arr[_i+1] ))
        if (( _p > _max )); then _max=$_p; fi
    done
    echo "$_max"
}