Wieferich pair
In mathematics, a Wieferich pair is a pair of prime numbers "p" and "q" that satisfy Wieferich pairs are named after German mathematician Arthur Wieferich. Wieferich pairs play an important role in Preda Mihăilescu's 2002 proof of Mihăilescu's theorem (formerly known as Catalan's conjecture). Known Wieferich pairs. There are only 7 Wieferich pairs known: Wieferich triple. A Wieferich triple is a triple of prime numbers "p", "q" and "r" that satisfy There are 17 known Wieferich triples: Barker sequence. Barker sequence or Wieferich "n"-tuple is a generalization of Wieferich pair and Wieferich triple. It is  primes ("p", "p", "p", ..., "p") such that For example, (3, 11, 71, 331, 359) is a Barker sequence, or a Wieferich 5-tuple; (5, 188748146801, 453029, 53, 97, 76704103313, 4794006457, 12197, 3049, 41) is a Barker sequence, or a Wieferich 10-tuple. For the smallest Wieferich "n"-tuple, see , for the ordered set of all Wieferich tuples, see . Wieferich sequence. Wieferich sequence is a special type of Barker sequence. Every integer "k">1 has its own Wieferich sequence. To make a Wieferich sequence of an integer "k">1, start with a(1)="k", a("n") = the smallest prime "p" such that a("n"-1) = 1 (mod  p) but a("n"-1) ≠ 1 or -1 (mod "p"). It is a conjecture that every integer "k">1 has a periodic Wieferich sequence. For example, the Wieferich sequence of 2: The Wieferich sequence of 83: The Wieferich sequence of 59: (this sequence needs more terms to be periodic) However, there are many values of a(1) with unknown status. For example, the Wieferich sequence of 3: The Wieferich sequence of 14: The Wieferich sequence of 39: It is unknown that values for "k" exist such that the Wieferich sequence of "k" does not become periodic. Eventually, it is unknown that values for  k exist such that the Wieferich sequence of "k" is finite. When a("n" - 1)="k", a("n") will be (start with "k" = 2): 1093, 11, 1093, 20771, 66161, 5, 1093, 11, 487, 71, 2693, 863, 29, 29131, 1093, 46021, 5, 7, 281, ?, 13, 13, 25633, 20771, 71, 11, 19, ?, 7, 7, 5, 233, 46145917691, 1613, 66161, 77867, 17, 8039, 11, 29, 23, 5, 229, 1283, 829, ?, 257, 491531, ?, ... (For "k" = 21, 29, 47, 50, even the next value is unknown)