OFFSET
1,3
COMMENTS
This is also a rectangular array read by rows, with four columns, in which T(j,k) is the number of cells (also the area) of the j-th gap between the arms in the k-th quadrant of the spiral of the symmetric representation of sigma described in A239660, with j >= 1 and 1 <= k <= 4 and starting with T(1,1) = 0, see example.
We can find the spiral (mentioned above) on the terraces of the stepped pyramid described in A244050. - Omar E. Pol, Dec 07 2016
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
a(5) = sigma(4) + sigma(3) + sigma(2) = 7 + 4 + 3 = 14. On the other hand a(5) = A024916(4) - A024916(1) = 15 - 1 = 14.
...
Also, if written as a rectangular array T(j,k) with four columns the sequence begins:
0, 1, 4, 8;
14, 17, 25, 26;
35, 36, 46, 43;
58, 54, 66, 62;
79, 73, 88, 77;
101, 94, 110, 92;
120, 115, 133, 113;
138, 126, 158, 134;
167, 143, 165, 150;
193, 177, 189, 154;
206, 188, 228, 182;
224, 206, 234, 198;
244, 229, 274, 222;
263, 224, 272, 246;
312, 272, 290, 230;
318, 290, 326, 262;
...
In this case T(2,1) = a(5) = 14.
MAPLE
L:= [0, 0, 0, seq(numtheory:-sigma(n), n=1..100)]:
L[1..101]+L[2..102]+L[3..103]; # Robert Israel, Dec 07 2016
MATHEMATICA
a252922[n_] := Block[{f}, f[1] = 0; f[2] = 1; f[3] = 4;
f[x_] := DivisorSigma[1, x - 1] + DivisorSigma[1, x - 2] +
DivisorSigma[1, x - 3]; Table[f[i], {i, n}]]; a252922[68] (* Michael De Vlieger, Dec 27 2014 *)
PROG
(PARI) v=concat([0, 1, 4], vector(100, n, sigma(n)+sigma(n+1)+sigma(n+2))) \\ Derek Orr, Dec 30 2014
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Omar E. Pol, Dec 24 2014
STATUS
approved