a(n) is the greatest m such that Sum_{k = 1..m} 1/(1 + n*k) <= 1.

Terms

a(0) =2a(1) =6a(2) =16a(3) =42a(4) =110a(5) =288a(6) =761a(7) =2020a(8) =5388a(9) =14417a(10) =38681a(11) =103994a(12) =280032a(13) =755031a(14) =2037848a(15) =5504884a(16) =14880978a(17) =40250609a(18) =108926101a(19) =294902398a(20) =798703663a(21) =2163878141