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A087269
Nonprime solutions to gcd(x, pi(x)) = gcd(x, A000720(x)) = 1.
3
1, 9, 12, 18, 21, 25, 26, 28, 32, 34, 35, 36, 42, 45, 49, 52, 55, 57, 60, 65, 68, 69, 70, 74, 76, 81, 84, 85, 86, 87, 88, 91, 95, 98, 99, 104, 106, 110, 111, 112, 119, 121, 128, 129, 130, 133, 135, 141, 143, 145, 147, 155, 158, 159, 160, 161, 162, 165, 170, 172, 177
OFFSET
1,2
COMMENTS
What is the density of this sequence? - David A. Corneth, Oct 21 2019
LINKS
EXAMPLE
There are 37 primes below the nonprime 162, so pi(162) = 37 and as gcd(162, pi(162)) = gcd(162, 37) = 1, 162 is in the sequence. - David A. Corneth, Oct 21 2019
MATHEMATICA
t=Table[GCD[w, PrimePi[w]], {w, 1, 1000}]; f=Flatten[Position[t, 1]]; cf=Part[f, Flatten[Position[PrimeQ[f], False]]]
PROG
(PARI) first(n) = {n = max(n, 2); my(q = 2, i = 1, t = 1, res = vector(n)); res[1] = 1; forprime(p = 3, oo, for(j = q + 1, p - 1, if(gcd(t, j) == 1, i++; if(i <= n, res[i] = j; , return(res); ) ) ); t++; q = p ) } \\ David A. Corneth, Oct 21 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Labos Elemer, Sep 16 2003
STATUS
approved