U(n) = R(6n)*R(n)/(R(3n)*R(2n))

U( 1) = 91 = 7*13
U( 2) = 9901 = prime
U( 3) = 999001 = 19*52579
U( 4) = 99990001 = prime
U( 5) = 9999900001 = 7*13*211*241*2161
U( 6) = 999999000001 = prime
U( 7) = 99999990000001 = 7^2*13*127*2689*459691
U( 8) = 9999999900000001 = prime
U( 9) = 999999999000000001 = 70541929*14175966169
U(10) = 99999999990000000001 = 61*9901*4188901*39526741
U(11) = 9999999999900000000001 = 7*13*599144041*183411838171
U(12) = 999999999999000000000001 = 3169*98641*3199044596370769
U(13) = 99999999999990000000000001 = 7*13^2*157*6397*216451*388847808493
U(14) = 9999999999999900000000000001 = 9901*226549*4458192223320340849
U(15) = 999999999999999000000000000001 = 19*29611*52579*3762091*8985695684401
U(16) = 99999999999999990000000000000001 = 97*206209*66554101249*75118313082913
U(17) = 9999999999999999900000000000000001 = 7*13*291078844423*377526955309799110357
U(18) = 999999999999999999000000000000000001 = 109*153469*59779577156334533866654838281
U(19) = 99999999999999999990000000000000000001 = 7*13*1458973*753201806271328462547977919407
U(20) = 9999999999999999999900000000000000000001 = 99990001*100009999999899989999000000010001
U(21) = 999999999999999999999000000000000000000001 = 19*52579*5274739*189772422673235585874485732659
U(22) = 99999999999999999999990000000000000000000001 = 9901*5419170769*789390798020221*2361000305507449
U(23) = 9999999999999999999999900000000000000000000001 = 7*13*31051*143574021480139*24649445347649059192745899
U(24) = 999999999999999999999999000000000000000000000001 = 8929*111994624258035614290513943330720125433979169
U(25) = 99999999999999999999999990000000000000000000000001 = 7*13*211*241*2161*10000099999999989999899999000000000100001
U(26) = 9999999999999999999999999900000000000000000000000001 = 3121*9901*53397071018461*6060517860310398033985611921721
U(27) = 999999999999999999999999999000000000000000000000000001 = 1459*2458921051*456502382570032651*610600386089858349939139
U(28) = 99999999999999999999999999990000000000000000000000000001 = 99990001*11189053009*603812429055411913*148029423400750506553
U(29) = 9999999999999999999999999999900000000000000000000000000001 = 7*13*638453709757*135080726389891*1274194732898148471766404179653
U(30) = 999999999999999999999999999999000000000000000000000000000001 = 181*999999000001*4999437541453012143121*1105097795002994798105101
U(31) = 99999999999999999999999999999990000000000000000000000000000001 = 7*13*373*44641*3590254957*18381907262281244633158190677786966663091011
U(32) = 9999999999999999999999999999999900000000000000000000000000000001 = 193*769*1253224535459902849*53763491189967221358575546107279034709697
U(33) = 999999999999999999999999999999999000000000000000000000000000000001 = 19*52579*7093127053*141122524877886182282233539317796144938305111168717
U(34) = 99999999999999999999999999999999990000000000000000000000000000000001 = 409*3061*9901*5969449*134703241*225974065503889*44398000479007997569751764249
U(35) = 9999999999999999999999999999999999900000000000000000000000000000000001 = 7^2*13*127*211*241*2161*2689*459691*29970369241*1661378260814161*18276168846821336356291
U(36) = 999999999999999999999999999999999999000000000000000000000000000000000001 = 1726290008991504500177463302688697*579276943498154282123686999881829009033