A177941 a(1)=5; for n>0, a(n+1)=a(n)+p-1, where p is the smallest prime divisor of (a(n))^2-4.

5, 7, 9, 15, 27, 31, 33, 37, 39, 75, 81, 159, 165, 327, 331, 333, 337, 339, 349, 351, 699, 715, 717, 721, 723, 727, 729, 745, 747, 751, 753, 757, 759, 1515, 1531, 1533, 1537, 1539, 1561, 1563, 1567, 1569, 3135, 3147, 3151, 3153, 3157, 3159, 3165, 6327, 6331, 6333, 6337

Offset: 1

Author

Vladimir Shevelev, May 15 2010

Keywords

• nonn

Crossrefs

Cf. A177929.

Programs

Maple

A020639 := proc(n) if n = 1 then 1 ; else numtheory[factorset](n) ; min(op(%)) ; end if; end proc:
A177941 := proc(n) option remember; if n = 1 then 5 else A020639((procname(n-1))^2-4) ; procname(n-1)+%-1 ; end if; end proc: seq(A177941(n),n=1..120) ; # R. J. Mathar, Jun 30 2010

Mathematica

NestList[# + FactorInteger[#^2 - 4][[1, 1]] - 1 &, 5, 52] (* or *)
a[1] = 5; a[n_] := a[n] = # + FactorInteger[#^2 - 4][[1, 1]] - 1 &@ a[n - 1]; Array[a, {53}] (* Michael De Vlieger, Feb 07 2016 *)

PARI

lista(nn) = {my(va = vector(nn)); va[1] = 5; for (n=2, nn, va[n] = va[n-1] + factor(va[n-1]^2-4)[1,1] - 1;); va;} \\ Michel Marcus, Dec 14 2018

Extensions

Entries checked by R. J. Mathar, Jun 30 2010