A351523 - OEIS

%I #30 Mar 13 2022 19:17:51

%S 4,6,12,30,60,180,630,1260,5040,27720,166320,1081080,8648640,73513440,

%T 698377680,8031343320,64250746560,803134332000,11243880648000,

%U 163036269396000,2527062175638000,30324746107656000,561007802991636000,11500659961328538000

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

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

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

%t 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 *)

%o (PARI) f(n) = {my(k=1); while (numdiv(n*k) != 2*numdiv(n), k++); n*k; } \\ A129902

%o 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

%Y Cf. A129902.

%K nonn

%O 1,1

%A _J. Lowell_, Feb 13 2022