Proof for OEIS A373225 Michael S. Branicky, May 30 2024 Remark: When p == 3 mod 4 is a term in A096448, it is a term of A373225 that is > 2. And vice versa. Proof of ⇒: Let p_n denote the nth prime. From A373223, we have T(n, k) = { 0, if n = k; { 1, if n = 1 or k = 1; { (-1)^(((p_n - 1)/2)*((p_k - 1)/2)), o.w. When p = p_n > 2, the row sum is Sum_{k=1..n} T(n, k) = T(n, 1) + T(n, 2) + ... + T(n, n-1) + T(n, n) = 1 + T(n, 2) + ... + T(n, n-1) + 0 = 1 + [(-1)^(((p_n-1)/2)*((p_2-1)/2))) + ... + (-1)^(((p_n - 1)/2)*((p_{n-1} - 1)/2)))] (*) When p_n = 3 mod 4, ((p_k-1)/2) is odd and the sign of each term is determined by whether (p_k-1)/2 is even or odd. When p_k = 1 mod 4, (p_k-1)/2 is even, so that term of the sum is 1; when p_k = 3 mod 4, (p_k-1)/2 is odd, so that term of the sum is -1. Now, suppose further that p_n is a term of A096448 ("Primes p such that the number of primes less than p equal to 1 mod 4 is one less than the number of primes less than p equal to 3 mod 4.") Then, the sum has one less 1 than -1, so the sum in brackets is -1 and (*) is 0. # Proof of ⇐: When p_n == 1 mod 4, ((p_n-1)/2) is even, so every term in the brackets is 1 and (*) cannot be 0. Thus, p_n == 3 mod 4 for terms > 2 in A373225. So, assume p_n == 3 mod 4, thus ((p_n-1)/2) is odd. Now, for (*) to be 0, we need the term in brackets to be -1. Since each term is -1 or 1, we need the number of -1's to be 1 more than the number of 1's, which happens when p_n is a member of A096448. #