OFFSET
1,2
COMMENTS
For any positive number with prime factorization Product_{k = 1..m} prime(k)^e_k (where prime(k) denotes the k-th prime number and e_m > 0), we build a XOR-triangle with (e_m, ..., e_1) as top row, and having each entry in the subsequent rows be the XOR of the two values above it. This sequence lists integers whose XOR-triangle has 3-fold rotational symmetry. A334990 gives the second row of such XOR-triangles for numbers that are not powers of 2.
This sequence has strong connections with A334556: for any n > 0 and k > 0, A019565(A334556(n))^k belongs to this sequence.
Every power of 2 belongs to the sequence.
If m belongs to this sequence, then m^2 also belongs to this sequence.
EXAMPLE
The number 15750 = 7^1 * 5^2 * 3^3 * 2^1 yields the following XOR-triangle:
1 2 3 1
3 1 2
2 3
1
As this XOR-triangle has rotational symmetry, 15750 belongs to this sequence.
PROG
(PARI) is(n) = {
my (e);
if (n==1, e=[],
my (f=factor(n), m=primepi(f[#f~, 1]));
e=vector(m, k, valuation(n, prime(m+1-k)))
);
my (x=e);
for (k=1, #e,
if (e[k]!=x[#x], return (0));
if (x[1]!=e[#e+1-k], return (0));
x=vector(#x-1, k, bitxor(x[k], x[k+1]));
);
return (1);
}
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Rémy Sigrist, May 19 2020
STATUS
approved