OFFSET
1,2
COMMENTS
The k-th triangular number t_k is given as t_k = k(k+1)/2. The t_k associated with this sequence form the intersection of A004613 and A000217.
Apart from 1, numbers whose prime factors are all congruent to 1 mod 4 are also known as primitive hypotenuse numbers because they are candidates for the hypotenuse of primitive right triangles.
For t_k to be a primitive hypotenuse number all its divisors must be congruent to 1 mod 4. Therefore k has to be odd and congruent to 1 mod 8.
LINKS
Frank M Jackson, Table of n, a(n) for n = 1..10000
EXAMPLE
a(2) = 25 because the 25th triangular number is 325, the prime factorization of 325 is 5^2*13, and 5,13 are both congruent to 1 mod 4. It is the second such occurrence.
MATHEMATICA
lst={}; Do[p=1+8n; If[Union@Mod[First/@FactorInteger[p(p+1)/2], 4]=={1}, AppendTo[lst, p]], {n, 0, 10^3}]; lst
PROG
(PARI) isok(k) = my(f=factor(k*(k+1)/2)[, 1]~); #select(x->((x%4)==1), f) == #f; \\ Michel Marcus, Nov 22 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Frank M Jackson, Nov 21 2020
STATUS
approved