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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.)