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 A260218 a(1) = 2; for n > 1 if n is even a(n) = spf(1 + Product_{odd m,m
 2, 3, 2, 5, 2, 3, 2, 17, 2, 3, 2, 5, 2, 3, 2, 257, 2, 3, 2, 5, 2, 3, 2, 17, 2, 3, 2, 5, 2, 3, 2, 65537, 2, 3, 2, 5, 2, 3, 2, 17, 2, 3, 2, 5, 2, 3, 2, 97, 2, 3, 2, 5, 2, 3, 2, 17, 2, 3, 2, 5, 2, 3, 2, 641, 2, 3, 2, 5, 2, 3, 2, 17, 2, 3, 2, 5, 2, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Antti Karttunen, Table of n, a(n) for n = 1..255 FORMULA It appears that for odd k, a(k) = 2 and for even k, a(k) = A002586(k/2). - Michel Marcus, Jul 20 2015 MATHEMATICA f[n_] := Block[{a = {2}, k, m}, Do[If[EvenQ@ k, AppendTo[a, FactorInteger[Product[a[[m]], {m, 1, k - 1, 2}] + 1][[1, 1]]], AppendTo[a, FactorInteger[Product[a[[m]], {m, 2, k - 1, 2}] + 1][[1, 1]]]], {k, 2, n}]; a]; f@ 80 (* Michael De Vlieger, Jul 20 2015 *) PROG (PARI) spf(n)=factor(n)[1, 1] first(m)=my(v=vector(m), i, odd=2, even=1); v[1]=2; for(i=2, m, if(i%2==0, v[i]=spf(odd+1); even*=v[i], v[i]=spf(even+1); odd*=v[i])); v; /* Anders Hellström, Jul 19 2015 */ (PARI) A020639(n) = if(1==n, n, vecmin(factor(n)[, 1])); memoA260218 = Map(); A260218(n) = if(1==n, 2, if(mapisdefined(memoA260218, n), mapget(memoA260218, n), my(k, m, v = if(!(n%2), k=1; m=1; while(k

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Last modified January 23 02:38 EST 2019. Contains 319365 sequences. (Running on oeis4.)