Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #10 Mar 06 2021 10:41:40
%S 1,1,4,2,64,59,1342,1076,4268,10739,64387,818998,3226976,26444853,
%T 53155423,1175849126,13396689992,2368807981,907313696551,483703442648,
%U 2835669288809,98713800566163
%N Smallest value of x such that x^2 + y^2 = P, where P is the product of the first n primes of the form 4k + 1, and 0 < x < y.
%e a(3) = 4 because the solutions to x^2 + y^2 = 5*13*17 are (x,y) = (23,24), (9,32), (4,33), (12,31) of which the smallest value of x is 4.
%o (PARI) prd(n) = my(q=3, t=1); for(k=1, n, until(q%4==1, q=nextprime(q+1)); t*=q); t; \\ A006278
%o a(n) = {my(t=prd(n), q=0); until(issquare(t-q^2), q++); q; } \\ _Michel Marcus_, Mar 05 2021
%Y Cf. A002144, A006278, A236382.
%K nonn,more
%O 1,3
%A _Colin Barker_, Jan 24 2014