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跟跑数列,收集在两个相继质数之间具有相同显著质因子的一组数, 但显著部分最大的那个数被排除.

This sequence, so called runner-up numbers, collects a set of numbers that have the same significant prime factor between two successive prime numbers, but the number with the largest significant part is excluded. Runner-up numbers are the integers i such that there exists j,prevprime(i)

Note:spf of 139..149 are 139, 7, 47, 71, 13, 2, 29, 73, 7, 37, 149
spf(140)==spf(147)==7
[619, 31, 3, 311, 89, 2, 5, 313, 19, 157, 37, 3,631]
spf(621)==spf(630)==3
[2503, 313, 167, 179, 109, 19, 193, 251, 3, 157, 359, 419, 503, 37, 839, 1259, 229, 3,2521]
spf(2511)==spf(2520)==3
so, spf(n) can NOT always as distinct prime factors for p[i]..p[i+1].