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A259828 a(1)=a(2)=2; thereafter, a(n)=gpf(1+a(n-1)a(n-2)), where gpf is greatest prime factor. 1
2, 2, 5, 11, 7, 13, 23, 5, 29, 73, 353, 859, 8423, 92761, 856717, 126948763, 122613509, 2102184467, 2530079, 2659346387041447, 334098941853251, 148080802321968921649521449033, 6863460080030077, 1678153631189331624730247, 7109839787546601453693131222417760271, 5808110138398623777046165714073 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..26.

MATHEMATICA

a = {2, 2}; Do[AppendTo[a, FactorInteger[1 + a[[n - 1]] a[[n - 2]]][[-1, 1]]], {n, 3, 26}]; a (* Michael De Vlieger, Aug 05 2015 *)

nxt[{a_, b_}] := {b, FactorInteger[a*b + 1][[-1, 1]]}; NestList[nxt, {2, 2}, 30][[All, 1]] (* Harvey P. Dale, Aug 03 2018 *)

PROG

(Sage)

def gpf(n):

    return (factor(n)[-1])[0]

def A259828vec(m): # m>2=f

    f=2

    v=[2, 2]

    for i in range(f, m):

        v.append(gpf(1+v[i-1]*v[i-2]))

    return v

(PARI) gpf(n)=my(f=factor(n)[, 1]); f[#f];

first(m)=my(v=vector(m)); v[1]=2; v[2]=2; for(i=3, m, v[i]=gpf(1+v[i-1]*v[i-2])); v;

CROSSREFS

Cf. A260725, A000945.

Sequence in context: A233018 A209100 A208864 * A104080 A078405 A109278

Adjacent sequences:  A259825 A259826 A259827 * A259829 A259830 A259831

KEYWORD

nonn

AUTHOR

Anders Hellström, Aug 05 2015

EXTENSIONS

Terms a(20)-a(26) from Michael De Vlieger, Aug 05 2015

STATUS

approved

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Last modified December 14 07:03 EST 2019. Contains 329978 sequences. (Running on oeis4.)