A351523 a(1)=4. For n > 1, a(n) = A129902(a(n-1))/2.

4, 6, 12, 30, 60, 180, 630, 1260, 5040, 27720, 166320, 1081080, 8648640, 73513440, 698377680, 8031343320, 64250746560, 803134332000, 11243880648000, 163036269396000, 2527062175638000, 30324746107656000, 561007802991636000, 11500659961328538000

Offset

1,1

Comments

The definition "a(1)=2, for n > 1, a(n) = A129902(a(n-1))/2" is simply 2 followed by all 3's.

Links

Table of n, a(n) for n=1..24.

Example

A129902(4)=12, and 12/2=6, so a(2)=6.

Mathematica

f[n_] := Module[{d = 2 * DivisorSigma[0, n], k = 2*n}, While[DivisorSigma[0, k] != d, k += n]; k]; a[1] = 4; a[n_] := a[n] = f[a[n - 1]]/2; Array[a, 24] (* Amiram Eldar, Feb 14 2022 *)

Prog

(PARI) f(n) = {my(k=1); while (numdiv(n*k) != 2*numdiv(n), k++); n*k; } \\ A129902
lista(nn) = {my(va = vector(nn)); va[1] = 4; for (n=2, nn, va[n] = f(va[n-1])/2; ); va; } \\ Michel Marcus, Feb 14 2022

Crossrefs

Cf. A129902.

Sequence in context: A068570 A092320 A056495 * A263656 A178674 A025018

Adjacent sequences: A351520 A351521 A351522 * A351524 A351525 A351526

Keyword

nonn

Author

J. Lowell, Feb 13 2022

Status

approved