A113929 - OEIS

A113929

Numbers k such that sigma(k) and phi(k) are both palindromes.

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

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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

Cf. A000010, A000203, A002113.

Sequence in context: A028986 A327324 A063948 * A220394 A082351 A122319

Adjacent sequences: A113926 A113927 A113928 * A113930 A113931 A113932

KEYWORD

base,nonn

AUTHOR

Giovanni Resta, Jan 30 2006

EXTENSIONS

a(21)-a(31) from Donovan Johnson, Dec 14 2009

STATUS

approved