A359642 - OEIS

%I #15 Jan 31 2025 16:11:13

%S 0,0,16,429,7039,92966,1103888,12364826,133702610,1413227318,

%T 14709861824,151469044739,1547593008310,15721130285808,

%U 159006397271949,1602820838558101,16114386617828822,161673560523193369,1619352576852638084,16197963371445222701

%N Number of numbers <= 10^n that are products of 4 distinct primes.

%e a(3) = 16 = #{210, 330, 390, 462, 510, 546, 570, 690, 714, 770, 798, 858, 870, 910, 930, 966}

%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, 4); \\ _Daniel Suteu_, Jan 11 2023

%Y Cf. A006880, A046386, A036351, A215218.

%K nonn,changed

%O 1,3

%A _Peter Dolland_, Jan 09 2023

%E a(14)-a(15) from _Daniel Suteu_, Jan 11 2023

%E a(16)-a(20) from _Henri Lifchitz_, Jan 31 2025