Follow-up to All but one bases of starting 5-tuples are Proth primes : u/No_Assist4814 and Bases of 5-tuples in domes follow the Swiss Watch (mod 100) : r/Collatz.
My new friend, Goggle AI, provides the following,
"Proth primes mod 100 do not form a single cyclic sequence because they are primes, but the underlying Proth numbers (k*2^n + 1\)) are governed by a strict 20-step cycle.
Because (2^n) (mod 100) has a repeating period of 20 starting at n=2, the last two digits of any Proth number with a fixed k will cycle through a predictable sequence every 20 powers.
The 20-Step Base Cycle
For any fixed odd multiplier k, the value of (2^n) (mod 100) loops through these 20 repeating remainders:
[4, 8, 16, 32, 64, 28, 56, 12, 24, 48, 96, 92, 84, 68, 36, 72, 44, 88, 76, 52]
Proth Endings Constraints
Multiplying this cycle by an odd k and adding 1 restricts the possible last two digits of all Proth numbers (and thus Proth primes) where n ≥ 2. They can only end in these 25 specific modular values:
- Ending in 1: 01, 21, 41, 61, 81
- Ending in 3: 13, 33, 53, 73, 93
- Ending in 5: 05 (Only occurs when N=5; higher values are multiples of 5)
- Ending in 7: 17, 37, 57, 77, 97
- Ending in 9: 09, 29, 49, 69, 89
Because all Proth primes (except 3) satisfy N≡1 (mod 4), they can never end in 31, 51, 71, 91, 03, 23, 43, 63, 83, 25, 45, 65, 85, 07, 27, 47, 67, 87, 19, 39, 59, 79, or 99."
It has not found the Swiss watch yet (The Collatz procedure works like a Swiss watch (mod 100) : r/Collatz), but is very close. Note that the remaining numbers mentioned just above appear in blue-green bridge series, while those mentioned before do the same on the yellow bridge series.
Project "Tuples and segments" in 13 pages : r/Collatz