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A244411 Nonprimes n such that the product of its divisors is a palindrome. 4
1, 4, 22, 26, 49, 111, 121, 202, 1001, 1111, 2285, 10001, 10201, 11111, 100001, 1000001, 1001001, 1012101, 1100011, 1101011, 1109111, 1111111, 3069307, 10000001, 12028229, 12866669, 100000001, 101000101, 110000011, 110091011, 200010002, 10000000001, 10011111001 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Primes trivially satisfy this property and are therefore not included in the sequence.

Numbers n such that A136522(A007955(n)) = 1.

A number is in the intersection of A002778 and A001358 iff it is in this sequence.

a(31) > 2*10^8.

a(32) > 4*10^8. - Chai Wah Wu, Aug 25 2015

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..47 (terms < 3.5*10^11)

EXAMPLE

The divisors of 26 are 1,2,13,26. And 1*2*13*26 = 676 is a palindrome. Thus 26 is a member of this sequence.

PROG

(PARI) rev(n)={r=""; for(i=1, #digits(n), r=concat(Str(digits(n)[i]), r)); return(eval(r))}

for(n=1, 2*10^8, if(!isprime(n), d=divisors(n); ss=prod(j=1, #d, d[j]); if(ss==rev(ss), print1(n, ", "))))

(Python)

import sympy

from sympy import isprime

from sympy import divisors

def rev(n):

..r = ""

..for i in str(n):

....r = i + r

..return int(r)

def a():

..for n in range(1, 10**8):

....if not isprime(n):

......p = 1

......for i in divisors(n):

........p*=i

......if rev(p)==p:

........print(n, end=', ')

a()

(Python)

from sympy import divisor_count, sqrt

A244411_list = [1]

for n in range(1, 10**5):

    d = divisor_count(n)

    if d > 2:

        q, r = divmod(d, 2)

        s = str(n**q*(sqrt(n) if r else 1))

        if s == s[::-1]:

            A244411_list.append(n) # Chai Wah Wu, Aug 25 2015

CROSSREFS

Cf. A007955, A136522, A028980, A002778, A001358.

Sequence in context: A009925 A059653 A022385 * A213240 A279314 A006753

Adjacent sequences:  A244408 A244409 A244410 * A244412 A244413 A244414

KEYWORD

nonn,base,hard

AUTHOR

Derek Orr, Jun 27 2014

EXTENSIONS

a(31) from Chai Wah Wu, Aug 25 2015

a(32)-a(33) from Giovanni Resta, Sep 20 2019

STATUS

approved

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Last modified April 5 06:13 EDT 2020. Contains 333238 sequences. (Running on oeis4.)