Numbers k which when sandwiched between two 4's give a multiple of k.

Terms

a(0) =1a(1) =2a(2) =4a(3) =11a(4) =13a(5) =14a(6) =22a(7) =26a(8) =28a(9) =44a(10) =52a(11) =77a(12) =91a(13) =137a(14) =146a(15) =274a(16) =292a(17) =548a(18) =9091a(19) =19802a(20) =39604a(21) =909091a(22) =5882353a(23) =10989011a(24) =12987013a(25) =13986014a(26) =15037594a(27) =16194332a(28) =19138756a(29) =21978022