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A113929
Numbers k such that sigma(k) and phi(k) are both palindromes.
1
1, 2, 3, 4, 5, 7, 2881, 15456, 20930, 26461, 26772, 43262, 135536, 271171, 2118161, 2362081, 2545951, 2779321, 4236322, 6354483, 12936656, 28666681, 221782512, 253676851, 259202401, 259828451, 276025121, 276949721, 437593059, 472911836
OFFSET
1,2
COMMENTS
phi(k) = A000010(k) is the Euler totient function, while sigma(k) = A000203(k) is the sum of divisors of k.
LINKS
David A. Corneth, Table of n, a(n) for n = 1..47 (terms <= 10^14 using Max Alekseyev's invphi)
EXAMPLE
sigma(2118161) = 2122212 and phi(2118161) = 2114112.
MATHEMATICA
Select[Range[473*10^6], AllTrue[{DivisorSigma[1, #], EulerPhi[ #]}, PalindromeQ]&] (* Harvey P. Dale, Aug 30 2021 *)
PROG
(PARI) ispal(n) = my(d=digits(n)); d == Vecrev(d) \\
forfactored(i=1, 10^10, if(ispal(eulerphi(i))&&ispal(sigma(i)), print1(i[1], ", "))) \\ Alexandru Petrescu, Jun 03 2022
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Giovanni Resta, Jan 30 2006
EXTENSIONS
a(21)-a(31) from Donovan Johnson, Dec 14 2009
STATUS
approved