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A252922
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a(n) = sigma(n-1) + sigma(n-2) + sigma(n-3), with a(1)=0, a(2)=1, a(3)=4.
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2
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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, 327, 315, 355, 296
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OFFSET
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1,3
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COMMENTS
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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
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LINKS
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FORMULA
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a(1) = 0, a(2) = sigma(1) = 1, a(3) = sigma(2) + sigma(1) = 4; for n >= 4, a(n) = sigma(n-1) + sigma(n-2) + sigma(n-3).
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EXAMPLE
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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.
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MAPLE
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L:= [0, 0, 0, seq(numtheory:-sigma(n), n=1..100)]:
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MATHEMATICA
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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 *)
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PROG
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(PARI) v=concat([0, 1, 4], vector(100, n, sigma(n)+sigma(n+1)+sigma(n+2))) \\ Derek Orr, Dec 30 2014
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CROSSREFS
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Cf. A000203, A010883, A024916, A092403, A112610, A193553, A196020, A236104, A237270, A237271, A237593, A239052, A239053, A239931-A239934, A239660, A240020, A244050, A245092, A262626.
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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