Math in Bash

From Augix' Wiki

[eva001:~/Work/cDNA/map13n augix] i=5000
[eva001:~/Work/cDNA/map13n augix] echo $i
5000
[eva001:~/Work/cDNA/map13n augix] let j=$i-5000
[eva001:~/Work/cDNA/map13n augix] echo $j
0
[yan001: clustalw] a=$(( 1*100+1 ))
[yan001: clustalw] echo $a
101