A036325 Composite numbers whose prime factors have no digits other than 8 and 9.
7921, 704969, 800911, 8001011, 8009021, 8802011, 8810911, 8899021, 62742241, 71281079, 79120021, 80001121, 80982001, 88109911, 88910021, 712089979, 712802869, 783378979, 784171079, 791120021, 791200121, 792012869, 800020021, 800109911, 800901121, 800991011, 809001101, 809811011, 880111121
Offset: 1
Examples
7921 is in the sequence because it's composite and its only prime factor is 89, only having digits 8 or 9. - _David A. Corneth_, Apr 30 2018
Links
- David A. Corneth, Table of n, a(n) for n = 1..10000 (first 5375 terms from Robert Israel)
- Index entries for sequences related to prime factors.
Programs
-
Maple
N:= 9: # to get all terms of <= N digits R:= 10^N: G:= {9}: S:= {1}: for n from 1 to N-1 do G:= map(t -> (t+8*10^n,t+9*10^n), G); newprimes:= select(isprime, G); for p in newprimes do S:= map(s -> seq(s*p^i,i=0..floor(log[p](R/s))), S) od od: sort(convert(remove(isprime, S minus {1}),list)); # Robert Israel, Apr 30 2018
Formula
Sum_{n>=1} 1/a(n) = Product_{p in A020472} (p/(p - 1)) - Sum_{p in A020472} 1/p - 1 = 0.0001296249159... . - Amiram Eldar, May 22 2022
Extensions
More terms from Robert Israel, Apr 29 2018
Comments