Empirical: a(n) = (1/403291461126605635584000000)*n^26 - (1/1477258099364855808000000)*n^25 + (1/7954466688887685120000)*n^24 - (3209/206816133911079813120000)*n^23 + (131927/89920058222208614400000)*n^22 - (861481/8174550747473510400000)*n^21 + (437537/74314097704304640000)*n^20 - (299549/1216451004088320000)*n^19 + (3029326739/430239513024921600000)*n^18 - (32397985427/430239513024921600000)*n^17 - (33761440499/6959756828344320000)*n^16 + (9756143223119/27839027313377280000)*n^15 - (47042070757114561/3619073550739046400000)*n^14 + (4302843471911021/13256679673036800000)*n^13 - (11627446648256317/2141463639490560000)*n^12 + (59122492602704257/1265410332426240000)*n^11 + (1747285196822704579/4889085375283200000)*n^10 - (523554005970618285661/26889969564057600000)*n^9 + (50790242697507746797291/153272826515128320000)*n^8 - (11090922706417930933457/3649353012264960000)*n^7 + (3838249723940212547237021/390278030478336000000)*n^6 + (10898113598547487702043023/97569507619584000000)*n^5 - (796776335871678031546393/498688594500096000)*n^4 + (987831359995286363556121/124672148625024000)*n^3 - (14743224214741931778449/1929449919196800)*n^2 - (312512882189774803/4461857400)*n + 177747871 for n>18