Numbers k which when sandwiched between two 3's give a multiple of k.

Terms

a(0) =1a(1) =3a(2) =11a(3) =13a(4) =21a(5) =33a(6) =39a(7) =77a(8) =91a(9) =137a(10) =219a(11) =411a(12) =9091a(13) =29703a(14) =909091a(15) =5882353a(16) =10989011a(17) =12145749a(18) =12987013a(19) =14354067a(20) =20979021a(21) =22556391a(22) =32967033a(23) =38961039a(24) =52631579a(25) =76923077a(26) =90909091a(27) =297029703a(28) =1185770751a(29) =2479338843