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A326380 Numbers m such that beta(m) = tau(m)/2 where beta(m) is the number of Brazilian representations of m and tau(m) is the number of divisors of m. 9
7, 13, 15, 21, 26, 40, 43, 57, 62, 73, 80, 85, 86, 91, 93, 111, 114, 124, 127, 129, 133, 146, 157, 170, 171, 172, 183, 211, 215, 219, 222, 228, 241, 242, 259, 266, 285, 292, 307, 312, 314, 333, 341, 343, 365, 366, 381, 399, 421, 422, 438, 444, 455, 463, 468, 471, 482, 507, 518, 532, 549, 553, 555, 585, 601, 614, 624 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

As tau(m) = 2 * beta(m), the terms of this sequence are not squares. Indeed, there are 3 subsequences which realize a partition of this sequence (see examples):

1) Non-oblong composites which have only one Brazilian representation with three digits or more, they form A326387.

2) Oblong numbers that have exactly two Brazilian representations with three digits or more; these oblong integers are a subsequence of A167783 and form A326385.

3) Brazilian primes for which beta(p) = tau(p)/2 = 1, they are in A085104 \ {31, 8191}.

LINKS

Table of n, a(n) for n=1..67.

Index entries for sequences related to Brazilian numbers

EXAMPLE

One example for each type:

15 = 1111_2 = 33_4 with tau(15) = 4 and beta(15) = 2.

3906 = 62 * 63 = 111111_5 = 666_25 = (42,42)_86 = (31,31)_125 = (21,21)_185 = (18,18)_216 = (14,14)_278 = 99_433 = 77_557 = 66_650 = 33_1301 = 22_1952, so tau(3906) = 24 with beta(3906) = 12.

43 = 111_6 is Brazilian prime, so tau(43) = 2 and beta(43) = 1.

PROG

(PARI) beta(n) = sum(i=2, n-2, #vecsort(digits(n, i), , 8)==1); \\ A220136

isok(n) = beta(n) == numdiv(n)/2; \\ Michel Marcus, Jul 03 2019

CROSSREFS

Cf. A000005 (tau), A220136 (beta).

Cf. A085104 (Brazilian primes).

Subsequence of A167782.

Cf. A326378 (tau(m)/2 - 2), A326379 (tau(m)/2 - 1), A326381 (tau(m)/2 + 1), A326382 (tau(m)/2 + 2), A326383 (tau(m)/2 + 3).

Sequence in context: A224773 A076196 A167782 * A257521 A053696 A090503

Adjacent sequences:  A326377 A326378 A326379 * A326381 A326382 A326383

KEYWORD

nonn,base

AUTHOR

Bernard Schott, Jul 03 2019

STATUS

approved

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Last modified April 4 05:34 EDT 2020. Contains 333212 sequences. (Running on oeis4.)