Find a formula that generates the following sequence a $1,$ a $2,$ a $3....$

a$)5,9,13,17,21,...$

b$)15,20,25,30,35,...$

c$)1,0\mathrm{.}9,0\mathrm{.}8,0\mathrm{.}7,0\mathrm{.}6,...$

d$)10,7,4,1,2,...$

e$)0\mathrm{.}5,1,1\mathrm{.}5,2,2\mathrm{.}5,...$