A359645 - OEIS

%I #16 Feb 01 2025 10:36:34

%S 0,0,0,0,20,1235,32396,605939,9446284,131733664,1706815354,

%T 21008871506,249145286508,2873325692759,32433194803107,

%U 359960491516138,3941261642520039,42679704453671033,457980431402674541

%N Number of numbers <= 10^n that are products of 6 distinct primes.

%e a(5) = 20 = #{30030, 39270, 43890, 46410, 51870, 53130, 62790, 66990, 67830, 71610, 72930, 79170, 81510, 82110, 84630, 85470, 91770, 94710, 98670, 99330}.

%o (PARI) a(n) = my(N=10^n); (f(m, p, k, j=1)=my(s=sqrtnint(N\m, k), count=0); if(k==2, forprime(q=p, s, count += primepi(N\(m*q)) - j; j+=1); return(count)); forprime(q=p, s, count += f(m*q, q+1, k-1, j+1); j+=1); count); f(1, 2, 6); \\ _Daniel Suteu_, Jan 11 2023

%Y Cf. A006880, A036351, A067885, A215218, A359642, A359644.

%K nonn,more

%O 1,5

%A _Peter Dolland_, Jan 09 2023

%E a(13)-a(14) from _Daniel Suteu_, Jan 11 2023

%E a(15)-a(19) from _Henri Lifchitz_, Feb 1 2025