A373973 - OEIS

%I #8 Jun 24 2024 13:56:09

%S 0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,1,1,0,0,0,0,0,0,

%T 1,1,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,1,0,1,0,0,0,0,0,0,1,0,1,0,0,0,

%U 0,1,0,1,0,0,0,0,0,0,0,0,1,0,0,1,0,0,0,0,0,1,1,0,0,0,0,0,0,0,1,0,0,0,0,0,1,0,0,1,0,0,0,1,0,0,0,0,1,0,0,0,0,0,0,0,0,1

%N a(n) = 1 if n can be expressed as (a^3 + b^3)/(a*b) with b > a >= 1, otherwise 0.

%C a(n) = 1 if n has a divisor that is a sum of 2 distinct positive cubes (i.e., is one of the terms of A024670), otherwise 0.

%H Antti Karttunen, <a href="/A373973/b373973.txt">Table of n, a(n) for n = 1..100080</a>

%H <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>

%F a(n) = signum(A373974(n)) = [A373974(n) > 0], where [ ] is the Iverson bracket.

%o (PARI)

%o A373971(n) = if(0==n,n,for(i=ceil(sqrtn(n\2+1, 3)), sqrtn(n-(1/2), 3), if(ispower(n-(i^3), 3), return(1))); 0); \\ After _M. F. Hasler_'s Apr 12 2008 program in A024670.

%o A373973(n) = { fordiv(n,d,if(A373971(d),return(1))); 0; };

%Y Characteristic function of A359225.

%Y Cf. A024670, A373971, A373974.

%K nonn

%O 1

%A _Antti Karttunen_, Jun 24 2024