Numbers k which when sandwiched between two 5's give a multiple of k.

Terms

a(0) =1a(1) =5a(2) =11a(3) =13a(4) =35a(5) =55a(6) =65a(7) =77a(8) =91a(9) =137a(10) =365a(11) =685a(12) =9091a(13) =49505a(14) =909091a(15) =5882353a(16) =10989011a(17) =12987013a(18) =20242915a(19) =23923445a(20) =34965035a(21) =37593985a(22) =52631579a(23) =54945055a(24) =64935065a(25) =76923077a(26) =90909091a(27) =495049505a(28) =1976284585a(29) =4132231405