Composite numbers with only prime proper prefixes and suffixes in base 10.

Terms

a(0) =22a(1) =25a(2) =27a(3) =32a(4) =33a(5) =35a(6) =52a(7) =55a(8) =57a(9) =72a(10) =75a(11) =77a(12) =237a(13) =297a(14) =537a(15) =597a(16) =713a(17) =717a(18) =737a(19) =2337a(20) =2397a(21) =2937a(22) =3113a(23) =3173a(24) =5937a(25) =5997a(26) =7197a(27) =7337a(28) =7397a(29) =29397