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A338039 Numbers m such that A338038(m) = A338038(A004086(m)) where A004086(i) is i read backwards and A338038(i) is the sum of the primes and exponents in the prime factorization of i ignoring 1-exponents; palindromes and multiples of 10 are excluded. 4
18, 81, 198, 576, 675, 819, 891, 918, 1131, 1304, 1311, 1818, 1998, 2262, 2622, 3393, 3933, 4031, 4154, 4514, 4636, 6364, 8181, 8749, 8991, 9478, 12441, 14269, 14344, 14421, 15167, 15602, 16237, 18018, 18449, 18977, 19998, 20651, 23843, 24882, 26677, 26892, 27225 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Palindromes (A002113) are excluded from the sequence because they obviously satisfy the condition.

Sequence is infinite since it includes 18, 1818, 181818, .... See link.

There are many cases of terms that are the repeated concatenation of integers like: 1818, 8181, 181818, ... , but also 131313131313131313131313131313 and more. See A338166.

If n is in the sequence and has d digits, and gcd(n, x) = gcd(A004086(n), x) where x = (10^((k+1)*d)-1)/(10^d-1), then the concatenation of k copies of n is also in the sequence. - Robert Israel, Oct 13 2020

LINKS

Michel Marcus, Table of n, a(n) for n = 1..2998

Daniel Tsai, A recurring pattern in natural numbers of a certain property, arXiv:2010.03151 [math.NT], 2020.

EXAMPLE

For m = 18 = 2*3^2, A338038(18) = 2 + (3+2) = 7 and for m = 81 = 3^4, A338038(81) = 7, so 18 and 81 are terms.

MAPLE

rev:= proc(n) local L, i;

  L:= convert(n, base, 10);

  add(L[-i]*10^(i-1), i=1..nops(L))

end proc:

g:= proc(n) local t;

  add(t[1]+t[2], t=subs(1=0, ifactors(n)[2]))

end proc:

filter:= proc(n) local r;

  if n mod 10 = 0 then return false fi;

  r:= rev(n);

  r <> n and g(r)=g(n)

end proc:

select(filter, [$1..30000]); # Robert Israel, Oct 13 2020

MATHEMATICA

s[1] = 0; s[n_] := Plus @@ First /@ (f = FactorInteger[n]) + Plus @@ Select[Last /@ f, # > 1 &]; Select[Range[30000], !Divisible[#, 10] && (r = IntegerReverse[#]) != # &&  s[#] == s[r] &] (* Amiram Eldar, Oct 08 2020 *)

PROG

(PARI) f(n) = my(f=factor(n)); vecsum(f[, 1]) + sum(k=1, #f~, if (f[k, 2]!=1, f[k, 2])); \\ A338038

isok(m) = my(r=fromdigits(Vecrev(digits(m)))); (m % 10) && (m != r) && (f(r) == f(m));

CROSSREFS

Cf. A004086 (read n backwards), A002113, A029742 (non-palindromes), A338038, A338166.

Sequence in context: A039408 A043231 A044011 * A085504 A214531 A271502

Adjacent sequences:  A338036 A338037 A338038 * A338040 A338041 A338042

KEYWORD

nonn,base

AUTHOR

Michel Marcus, Oct 08 2020

STATUS

approved

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Last modified July 24 05:05 EDT 2021. Contains 346273 sequences. (Running on oeis4.)