A087269 -id:A087269 - cp's OEIS frontend

A087270 Solutions to gcd(x,pi(x)) = gcd(x, A000720(x)) > 1. Numbers x such that x and pi(x) have common divisor larger than one.

4, 6, 8, 10, 14, 15, 16, 20, 22, 24, 27, 30, 33, 38, 39, 40, 44, 46, 48, 50, 51, 54, 56, 58, 62, 63, 64, 66, 72, 75, 77, 78, 80, 82, 90, 92, 93, 94, 96, 100, 102, 105, 108, 114, 115, 116, 117, 118, 120, 122, 123, 124, 125, 126, 132, 134, 136, 138, 140, 142, 144, 146

Offset: 1

Labos Elemer, Sep 16 2003

nonn

Cf. A007053, A000720, A087266-A087269, A057809.

t=Table[GCD[w, PrimePi[w]], {w, 1, 1000}]; Flatten[Position[Sign[t-1], 1]]

A087271 Least number x such that gcd(x, pi(x)) = n.

Original entry on oeis.org

1, 4, 6, 8, 50, 66, 77, 56, 27, 30, 33, 156, 169, 182, 465, 224, 238, 252, 2299, 1380, 189, 902, 207, 96, 100, 1872, 1323, 2464, 1247, 120, 1333, 3168, 528, 1258, 1295, 828, 3441, 2888, 1755, 5800, 1271, 1932, 731, 748, 765, 2852, 2209, 11568, 2695, 4000

Offset: 1

Labos Elemer, Sep 16 2003

nonn

			n=253: a(253)=91586, pi(91586)=8855,
gcd(91586, 8855) = 253 first time.

Cf. A007053, A000720, A087266-A087269, A057809.

f[x_] := GCD[x, PrimePi[x]]; t=Table[0, {257}]; Do[s=f[n]; If[s<258&&t[[s]]==0, t[[s]]=n], {n, 1, 100000}]; t
Module[{tbl=Table[{x,GCD[x,PrimePi[x]]},{x,12000}]},Table[SelectFirst[ tbl,#[[2]]==n&],{n,50}]][[All,1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Feb 12 2020 *)

a(n) = Min{x; gcd(x, A000720(x))=n}.

cp's OEIS Frontend

A087270 Solutions to gcd(x,pi(x)) = gcd(x, A000720(x)) > 1. Numbers x such that x and pi(x) have common divisor larger than one.

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