OFFSET
1,1
COMMENTS
Also nontriangular numbers with digital root in {2, 4, 5, 7, 8}.
REFERENCES
Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966. See p. 190.
LINKS
Stefano Spezia, Table of n, a(n) for n = 1..10000
Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,1,-1).
FORMULA
G.f.: x*(2 + 2x + x^2 + 2*x^3 + x^4 + x^5)/((1 - x)^2*(1 + x + x^2 + x^3 + x^4)).
a(n) ~ 9*n/5.
MATHEMATICA
PROG
(PARI) list(lim)=my(v=List()); forstep(n=2, lim\1, [2, 1, 2, 1, 3], listput(v, n)); Vec(v) \\ Charles R Greathouse IV, Aug 07 2024
(PARI) is(n)=!setsearch([0, 1, 3, 6], n%9) \\ Charles R Greathouse IV, Aug 07 2024
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
Stefano Spezia, Aug 07 2024
STATUS
approved