an observation...

[Ralf Stephan]

The first sequence seems a subset of the second. Is this trivial?


%S A016105 21,33,57,69,77,93,129,133,141,161,177,201,209,213,217,237,249,253,301,
%N A016105 Blum numbers: of form P*Q where P&Q are distinct primes congruent to 3 (mod 4).

%S A084109 21,33,57,69,77,93,105,129,133,141,161,165,177,189,201,209,213,217,237,
%N A084109 n is congruent to 1 (mod 4) and is not the sum of two squares.
%C A084109 Alternatively, n is congruent to 1 (mod 4) such that no prime factor in the square-free part of n is congruent to 3 (mod 4).
%C A084109 Applications to the theory of optimal weighing designs and maximal determinants: An (n+1) X (n+1) conference matrix is impossible.
%C A084109 The upper bound of Ehlich/Wojtas on the determinant of a (0,1) matrix of order congruent to 1 (mod 4) cannot be achieved for n X n matrices.
%C A084109 The bound of Ehlich/Wojtas on the determinant of a (-1,1) matrix of order congruent to 2 (mod 4) cannot be achieved for (n+1) X (n+1) matrices.



ralf