1,1

Each term k, doubled, can be put into a one-to-one correspondence with a maximal Schreier set (a subset of the positive integers with cardinality equal to the minimum element in the set) by interpreting the 1-based position of the ones in the binary expansion of 2*k (where position 1 corresponds to the least significant bit) as the elements of the corresponding maximal Schreier set. See A373556 for more information. Cf. also A371176. - Paolo Xausa, Jun 13 2024

a(n) = a(n-1) + b(n) for n > 1 with a(1) = 3 where b(n) = {2^(n-1) if n < 4; 5 if c(n-1) = 1; otherwise 2*b(n - A000045(A072649(n-1) + 1)) - [c(n) = 1]} and where c(n) = A010056(n).

A025480(a(n)-1) = A048679(n) for n > 0.

a(A000045(n)) = 2^(n-1) + 1 for n > 1.

Select[Range[500], DigitCount[#, 2, 1] == IntegerExponent[#, 2] + 2 &] (* Amiram Eldar, Jul 04 2022 *)

(PARI) r=quadgen(5);

A355489_upto(nMax)={my(v1, v2, v3, v4); v1=vector(nMax, i, 0); v1[1]=1; for(i=1, nMax-1, v1[i+1]=v1[i\r+1]+1); v2=vector(nMax, i, 0); v2[1]=1; for(i=2, nMax, v2[i]=v1[i]-v1[i-1]); v3=vector(nMax, i, 0); for(i=1, 3, v3[i]=2^(i-1)); for(i=4, nMax, v3[i]=if(v2[i-1]==1, 5, 2*v3[i-fibonacci(v1[i-1]+1)]-if(v2[i]==1, 1, 0))); v4=vector(nMax, i, 0); v4[1]=3; for(i=2, nMax, v4[i]=v4[i-1]+v3[i]); v4}

(PARI) isok(k) = hammingweight(k) == valuation(k, 2) + 2; \\ Michel Marcus, Jul 06 2022

(Python)

from itertools import count, islice

def A355489_gen(startvalue=1): # generator of terms >= startvalue

return filter(lambda n:n.bit_count()==(n&-n).bit_length()+1, count(max(startvalue, 1)))

A355480_list = list(islice(A355489_gen(), 30)) # Chai Wah Wu, Jul 15 2022

Cf. A000045, A000120, A007814, A010056, A025480, A048679, A072649, A371176, A373556.

Sequence in context: A211389 A335193 A288259 * A372639 A082874 A266250

Adjacent sequences: A355486 A355487 A355488 * A355490 A355491 A355492

nonn,base

Mikhail Kurkov, Jul 04 2022 [verification needed]

approved