Bash script using negative to cycle through -

i'm trying run bash script includes nested loop within variable should cycle through negative exponents, viz:

for abpow in {-11..-9} abc = $((10**$abpow))           abcoeff in {1..9}                         sed -e 's/acoeff/'$abc'/'\ 

this inner 2 loops of code. when values in first bracket (for abpow) positive, code runs fine. however, when have them above, need, error communicated screen is:

./scripting_test2.bash: line 30: 10**-11: exponent less 0 (error token "1") 

how make run? in advance.

ps: tried putting negative sign in front of $abpow exponents still recorded positive.

bash not support floating point arithmetic (which necessary raising negative power). instead, should use bc utility.

abc=$(bc -l <<< "10 ^($abpow)") 

also, there should no spaces before , after = in variable assignments