A356157 The nearest common ancestor of sigma(n) and gcd(n, sigma(n)) in the Doudna tree (A005940).

1, 1, 1, 1, 1, 6, 1, 1, 1, 2, 1, 2, 1, 2, 3, 1, 1, 3, 1, 2, 1, 2, 1, 6, 1, 2, 1, 28, 1, 2, 1, 1, 3, 2, 1, 1, 1, 2, 1, 3, 1, 6, 1, 2, 3, 2, 1, 2, 1, 1, 2, 2, 1, 6, 1, 2, 1, 2, 1, 3, 1, 2, 1, 1, 1, 2, 1, 2, 3, 2, 1, 3, 1, 2, 1, 2, 1, 3, 1, 2, 1, 2, 1, 28, 1, 2, 3, 2, 1, 2, 7, 2, 1, 2, 3, 3, 1, 1, 3, 1, 1, 2, 1, 2, 3

Offset: 1

Author

Antti Karttunen, Jul 30 2022

Keywords

• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 1..65537
• Index entries for sequences computed from indices in prime factorization
• Index entries for sequences related to sigma(n)

Crossrefs

Cf. A000203, A009194, A336702 (fixed points), A348040, A348041.

Cf. also A347879, A356156, A356307.

Programs

PARI

Abincompreflen(n, m) = { my(x=binary(n),y=binary(m),u=min(#x,#y)); for(i=1,u,if(x[i]!=y[i],return(i-1))); (u);};
Abinprefix(n,k) = { my(digs=binary(n)); fromdigits(vector(k,i,digs[i]),2); };
A005940(n) = { my(p=2, t=1); n--; until(!n\=2, if((n%2), (t*=p), p=nextprime(p+1))); (t); };
A156552(n) = {my(f = factor(n), p, p2 = 1, res = 0); for(i = 1, #f~, p = 1 << (primepi(f[i, 1]) - 1); res += (p * p2 * (2^(f[i, 2]) - 1)); p2 <<= f[i, 2]); res}; \\ From A156552
A348040sq(x,y) = Abincompreflen(A156552(x), A156552(y));
A348041sq(x,y) = A005940(1+Abinprefix(A156552(x),A348040sq(x,y)));
A356157(n) = A348041sq(sigma(n),gcd(n, sigma(n)));