Empirical: a(n) = (1/8222838654177922817725562880000000)*n^31 + (1/16578303738261941164769280000000)*n^30 + (1/69803384161102910167449600000)*n^29 + (283/130666433405020225929216000000)*n^28 + (12737/54444347252091760803840000000)*n^27 + (887/46149054674373181440000000)*n^26 + (2928493/2345294958551645080780800000)*n^25 + (308844191/4690589917103290161561600000)*n^24 + (67315849529/23452949585516450807808000000)*n^23 + (1387967249/13242772210907086848000000)*n^22 + (12327001/3808378936470011904000)*n^21 + (48631074923/571256840470501785600000)*n^20 + (793738418765489/414250468222437310464000000)*n^19 + (25831954623714997/697684999111473364992000000)*n^18 + (8585599512290443/13953699982229467299840000)*n^17 + (937742810289457/106598166403586457600000)*n^16 + (7183724560908031217/66446190391568891904000000)*n^15 + (2912894673607608377/2555622707368034304000000)*n^14 + (628771545873106265449/61181607614390741237760000)*n^13 + (1098289011870188756081/13904910821452441190400000)*n^12 + (51945102898825703311861/100627644102616350720000000)*n^11 + (392205511149607328407151/136566088424979333120000000)*n^10 + (286929167161489663810721/20940133558496831078400000)*n^9 + (330074814468483753883603/5816703766249119744000000)*n^8 + (5652854684675422239626089/27568752225451557120000000)*n^7 + (2132660136647815243157527/3341666936418370560000000)*n^6 + (1364274255930306587281/800835213520742400000)*n^5 + (7917292429292915797/2059290549053337600)*n^4 + (432330437257626224633/60430371469243776000)*n^3 + (10229641786922203/978217616376000)*n^2 + (4506616120609/412581579696)*n + 6 for n>15