OFFSET
1,1
COMMENTS
9 times the sum of the divisors of n.
a(n) is also the total number of horizontal rhombuses in the terraces of the n-th level of an irregular stepped pyramid (starting from the top) in which the structure of every 40-degree-three-dimensional sector arises after the 40-degree-zig-zag folding of every row of the diagram of the isosceles triangle A237593. The top of the pyramid is a nine-pointed star formed by nine rhombuses (see Links section).
LINKS
FORMULA
MAPLE
with(numtheory): seq(9*sigma(n), n=1..64);
MATHEMATICA
9*DivisorSigma[1, Range[70]] (* After Harvey P. Dale *)
PROG
(PARI) a(n) = 9 * sigma(n);
(GAP) List([1..70], n->9*Sigma(n)); # After Muniru A Asiru
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Jun 26 2019
STATUS
approved