Semiprimes (A001358) made of nontrivial runs of identical digits.

Terms

a(0) =22a(1) =33a(2) =55a(3) =77a(4) =111a(5) =1111a(6) =1133a(7) =1177a(8) =1199a(9) =3377a(10) =4411a(11) =4499a(12) =5533a(13) =5599a(14) =6611a(15) =6677a(16) =7711a(17) =7799a(18) =8899a(19) =9977a(20) =11111a(21) =11199a(22) =11333a(23) =11555a(24) =22233a(25) =22255a(26) =22299a(27) =22333a(28) =22999a(29) =33322