A242066 - OEIS

A242066

The smallest even k such that lpf_3(k-3) > lpf_3(k-1) >= p_n, where lpf_3(n) = lpf(n/3^t) (cf. A020639) such that 3^t (t>=0) is the maximal power of 3 which divides n.

16, 22, 34, 40, 70, 70, 70, 112, 112, 112, 130, 130, 142, 160, 184, 184, 202, 214, 310, 310, 310, 310, 310, 310, 310, 340, 340, 340, 382, 412, 412, 490, 490, 490, 490, 490, 502, 544, 544, 544, 574, 580, 634, 634, 634, 754, 754, 754, 754, 754, 754, 754, 772

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OFFSET

3,1

COMMENTS

a(n)-3 and (a(n)-1)/3 are primes.

LINKS

Table of n, a(n) for n=3..55.

MATHEMATICA

lpf[n_]:=lpf[n]=First[First[FactorInteger[n]]]; (*least prime factor*)

lpf3[n_]:=lpf3[n]=If[#==1, 1, lpf[#]]&[n/3^IntegerExponent[n, 3]];

Table[NestWhile[#+2&, 2, !(lpf3[#-3]>lpf3[#-1]>=Prime[n])&], {n, 3, 100}] (* Peter J. C. Moses, Aug 14 2014 *)

CROSSREFS