Numbers k which when sandwiched between two 7's give a multiple of k.

Terms

a(0) =1a(1) =7a(2) =11a(3) =13a(4) =49a(5) =77a(6) =91a(7) =137a(8) =511a(9) =959a(10) =9091a(11) =69307a(12) =909091a(13) =5882353a(14) =10989011a(15) =12987013a(16) =28340081a(17) =33492823a(18) =48951049a(19) =52631579a(20) =76923077a(21) =90909091a(22) =693069307a(23) =2766798419a(24) =4347826087a(25) =5785123967a(26) =9090909091a(27) =13698630137a(28) =51094890511a(29) =95890410959