A216606 -id:A216606 - cp's OEIS frontend

A319527 a(n) = 7 * sigma(n).

7, 21, 28, 49, 42, 84, 56, 105, 91, 126, 84, 196, 98, 168, 168, 217, 126, 273, 140, 294, 224, 252, 168, 420, 217, 294, 280, 392, 210, 504, 224, 441, 336, 378, 336, 637, 266, 420, 392, 630, 294, 672, 308, 588, 546, 504, 336, 868, 399, 651, 504, 686, 378, 840, 504, 840, 560, 630, 420, 1176, 434, 672, 728, 889

Offset: 1

Omar E. Pol, Sep 22 2018

7 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 (360/7)-degree-three-dimensional sector arises after the (360/7)-degree-zig-zag folding of every row of the diagram of the isosceles triangle A237593. The top of the pyramid is a seven-pointed star formed by seven rhombuses (see Links section).

k times sigma(n), k=1..8: A000203, A074400, A272027, A239050, A274535, A274536, this sequence, A319528.

Cf. A216606, A237593.

List([1..70],n->7*Sigma(n)); # Muniru A Asiru, Sep 28 2018

with(numtheory): seq(7*sigma(n), n=1..64);

7*DivisorSigma[1,Range[70]] (* Harvey P. Dale, Mar 14 2020 *)

PARI
```
a(n) = 7 * sigma(n);
```

a(n) = 7*A000203(n).
a(n) = A000203(n) + A274536(n).
Dirichlet g.f.: 7*zeta(s-1)*zeta(s). - (After Ilya Gutkovskiy)

A255873 The first nonzero digit of n/7.

Original entry on oeis.org

1, 2, 4, 5, 7, 8, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1

Offset: 1

Kit Scriven, Mar 08 2015

			The leading (most significant) digit of 22/7 in A068028 is 3, so a(22)=3.

Alois P. Heinz, Table of n, a(n) for n = 1..10000

Cf. A020806 (1/7), A068028 (22/7), A216606 (360/7).

A255873 := proc(n)
    local nshf ;
    nshf := n/7 ;
    while nshf < 1 do
        nshf := 10*nshf;
    end do;
    while nshf >= 10 do
        nshf := nshf/10;
    end do;
    floor(nshf) ;
end proc: # R. J. Mathar, May 28 2016

f[n_] := RealDigits[n/7, 10, 9][[1, 1]]; Array[f, 105] (* Robert G. Wilson v, Mar 08 2015 *)

a(n) = {my(x = n/7.0); if (x < 1, x *= 10); while (x >= 10, x /= 10); floor(x);} \\ Michel Marcus, Mar 12 2015

cp's OEIS Frontend

A319527 a(n) = 7 * sigma(n).

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