a(6)=2:132546142536。
a(7)=29:1243657,1253647,1324657,1325467,1325476,1326475,1326574,1425367,1425376,1426375,1426573,1436572,1526374,1526473,1536472,2143657,2153647,2436571,2536471,3142657,3152647,4132657,4152677,5132647,5142637,6132547,6142537,7132546,7142536。