A265395 Record values in A265388.

0, 6, 15, 33, 91, 435, 703, 1891, 2701, 12403, 18721, 38503, 49141, 79003, 88831, 104653, 146611, 188191, 218791, 226801, 269011, 286903, 385003, 497503, 597871, 665281, 721801, 736291, 765703, 873181, 954271, 1056331, 1314631, 1373653, 1537381, 1755001, 1869211, 1987021, 2035153, 2284453, 2741311, 3059101, 3270403, 3363121, 3913003

Offset

1,2

Comments

Question: Are all terms apart from 0 and 435 (= 3*5*29) semiprimes? See also comments in A265394 & A265396.

Links

Chai Wah Wu, Table of n, a(n) for n = 1..10000 (terms 1..192 from Antti Karttunen)

Formula

a(n) = A265388(A265394(n)).

Prog

(PARI) lista(nn) = {r = 0; for (n=1, nn, nr = gcd(vector(n-1, k, binomial(2*n, 2*k))); if (nr > r, print1(nr, ", "); r = nr); ); } \\ Michel Marcus, Dec 08 2015. Note: this gives correct value for a(1) only from PARI version 2.8 onward, see A265388 for why. - Antti Karttunen, Dec 11 2015
(Python)
from math import prod
from itertools import count, islice
from sympy.ntheory.factor_ import digits
from sympy import primefactors
def A265395_gen(): # generator of terms
    c = -1
    for n in count(1):
        m = prod((p if sum(digits(n<<1, p)[1:])==2 else 1) for p in primefactors(n*((n<<1)-1)) if p>2)<<(not(n&-n)^n) if n>1 else 0
        if m>c:
            yield m
            c = m
A265395_list = list(islice(A265395_gen(), 20)) # Chai Wah Wu, May 04 2026

Crossrefs

Cf. A265388, A265394, A265396.

Sequence in context: A051410 A083052 A333959 * A350596 A232604 A332735

Adjacent sequences: A265392 A265393 A265394 * A265396 A265397 A265398

Keyword

nonn

Author

Antti Karttunen, Dec 08 2015

Status

approved