Wednesday, 12 March 2025

The Good Prime

Yes, there is such a thing as a good prime and it is defined as follows:

A prime pn is said to be good if p2n>pnipn+i for all 1i<n.

The term was drawn to my attention because the prime associated with my diurnal age today (27737) and its earlier cousin prime (27733) are both good primes. The initial good primes are: 

5, 11, 17, 29, 37, 41, 53, 59, 67, 71, 97, 101, 127, 149, 179, 191, 223, 227, 251, 257, 269, 307, 311, 331, 347, 419, 431, 541, 557, 563, 569, 587, 593, 599, 641, 727, 733, 739, 809, 821, 853

Let's look at 17 as an example where 172=289. The primes less than it are 2, 3, 5, 7, 11 and 13. The corresponding primes larger than it are 43, 37, 31, 29, 23 and 19. So we have:2×43=86<2893×37=111<2895×31=155<2897×29=203<28911×23=253<28913×19=247<289

The earliest runs of 2, 3, 4, 5, 6 and 7 consecutive good primes start at 37, 557, 1847, 216703, 6929381, 134193727 and 15118087477. The good primes from 27733 to 40000 are as follows (permalink):

27733, 27737, 28277, 28387, 28403, 28493, 28537, 28571, 28591, 28597, 29833, 29983, 30011, 30059, 30089, 30491, 30631, 30637, 30671, 30757, 30803, 31121, 31139, 31147, 31957, 32027, 32051, 32057, 32297, 32969, 33287, 33311, 33329, 34123, 35729, 35747, 35797, 35801, 35831, 35951, 35963, 36433, 36451, 36467, 36523, 36527, 36671, 38113, 38149, 38167, 38177, 38543, 38557, 38593, 38651, 38669, 39079, 39089

Clearly the primes above and below the aspiring good prime need to be fairly bunched up, especially the ones above, and this is indeed the case for 27733 and 27737.

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