OFFSET
1,3
COMMENTS
Equivalently, numbers not of the form (x - y)^2*(x + y) or d^2*(2m + d), for (x, y) = (m+d, m). This shows that excluded are all squares d^2 > 0 times any number of the same parity and larger than d. In particular, for d=1, all odd numbers > 1, and for d=2, 4*(even numbers > 4) = 8*(odd numbers > 2). For larger d, no further (neither odd nor even) numbers are excluded.
So apart from 0, 1 and 8, this consists of even numbers not multiple of 8. All these numbers occur, since for larger (odd or even) d, no additional term is excluded.
FORMULA
Asymptotic density is 3/8.
a(n) = round((n-2)*9/8)*2 for all n > 6.
EXAMPLE
a(1) = 0, a(2) = 1 and a(3) = 2 obviously can't be of the form (x - y)(x^2 - y^2) with x > y > 0, which is necessarily greater than 1*3 = 3.
See A321499 for examples of the terms that are not in the sequence.
PROG
(PARI) is(n)={!n||!fordiv(n, d, d^2*(d+2)>n && break; n%d^2&&next; bittest(n\d^2-d, 0)||return)} \\ Uses the initial definition. More efficient variant below:
(PARI) select( is_A321501(n)=!bittest(n, 0)&&(n%8||n<9)||n<3, [0..99]) \\ Defines the function is_A321501(). The select() command is an illustration and a check.
(PARI) A321501_list(M)={setunion([1], setminus([0..M\2]*2, [2..M\8]*8))} \\ Return all terms up to M; more efficient than to use select(..., [0..M]) as above.
(PARI) A321501(n)=if(n>6, (n-2)*9\/8*2, n>3, n*2-4, n-1)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
M. F. Hasler, Nov 22 2018
STATUS
approved