Thank you for your gift to the thread, @CalbernandHowbe .
As a starting point -- merely to help "prime" the pump when pondering the problem (pun intended) -- one concrete example springs immediately to mind. Let p = 5. Then p-1 is a multiple of 4.
Furthermore, there exists the integer 7 such that 7^2 + 1 = 50 which is a multiple of our p = 5.
So we know that there exists at least ONE example that fits the prescription.
Furthermore, given the definition of divisibility, we know that our prime p must be in the form 4m + 1, where m is an integer.
We know, also, that (p-1) is a composite integer, not itself a prime -- since it is an integer by closure and since the prime gap is greater than 1 except for the gap between the first two primes (2 and 3), while neither of those primes fits the requirement that its integer predecessor be a multiple of 4.
We still have miles to go before we sleep, however. Which doesn't mean that the trip is ultimately a long one -- but I, at least, may wander for a time, unnecessarily, before finding the path. Sometimes we must wave our hand around a great deal before it appears in front of our face.
I wish that the integers were closed under the operation of taking a root. But, alas, they are not. Otherwise this would already be finished.
I start to mull over modular congruence and linear combinations and even mathematical induction, but those so far merely tantalize and have not brought immediate comprehension. Fortunately, I've learned to live comfortably with uncertainty. We cannot schedule our own epiphanies, when and if they arrive.
It is a lovely problem, though, and I simply wanted to get this down on the page, and to thank you, before proceeding, perhaps, to ponder productively.
Thank you for your gift to the thread, @CalbernandHowbe .
As a starting point -- merely to help "prime" the pump when pondering the problem (pun intended) -- one concrete example springs immediately to mind. Let p = 5. Then p-1 is a multiple of 4.
Furthermore, there exists the integer 7 such that 7^2 + 1 = 50 which is a multiple of our p = 5.
So we know that there exists at least ONE example that fits the prescription.
Furthermore, given the definition of divisibility, we know that our prime p must be in the form 4m + 1, where m is an integer.
We know, also, that (p-1) is a composite integer, not itself a prime -- since it is an integer by closure and since the prime gap is greater than 1 except for the gap between the first two primes (2 and 3), while neither of those primes fits the requirement that its integer predecessor be a multiple of 4.
We still have miles to go before we sleep, however. Which doesn't mean that the trip is ultimately a long one -- but I, at least, may wander for a time, unnecessarily, before finding the path. Sometimes we must wave our hand around a great deal before it appears in front of our face.
I wish that the integers were closed under the operation of taking a root. But, alas, they are not. Otherwise this would already be finished.
I start to mull over modular congruence and linear combinations and even mathematical induction, but those so far merely tantalize and have not brought immediate comprehension. Fortunately, I've learned to live comfortably with uncertainty. We cannot schedule our own epiphanies, when and if they arrive.
It is a lovely problem, though, and I simply wanted to get this down on the page, and to thank you, before proceeding, perhaps, to ponder productively.