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A257897 Numbers n such that n = concat(a,b) and n | a^b + b^a , with a>0 and b>0. 2
63, 103, 128, 147, 155, 212, 272, 292, 351, 452, 486, 497, 525, 527, 537, 584, 607, 624, 648, 729, 979, 999, 1024, 1296, 1323, 1359, 1533, 1541, 1575, 1809, 1872, 2048, 2050, 2107, 2187, 2448, 2512, 2537, 2564, 2763, 2793, 2886, 3072, 3357, 3927, 4096, 4263, 4284 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

We can have different solutions for the same number. E.g.: 2048 divides both (20^48 + 48^20) and (204^8 + 8^204). The same occurs for 4096, 4263, 16807, 32768, 96957, 156672, 186624, 252081, 262144, 270729, 352947, 390624 … The first number with 3 different concatenations is 186624 that divides (18^6624 + 6624^18), (186^624 + 624^186) and (1866^24 + 24^1866).

LINKS

Paolo P. Lava, Table of n, a(n) for n = 1..400

EXAMPLE

6^3 + 3^6 = 945 and 945 / 63 = 15;

10^3 + 3^10 = 60049 and 60049 / 103 = 583;

12^8 + 8^12 = 69149458432 and 69149458432 / 128 = 540230144; etc.

MAPLE

with(numtheory); P:=proc(q) local a, b, i, n; for n from 1 to q do

for i from 1 to ilog10(n) do a:=trunc(n/10^i); b:=n-a*10^i;

if a>0 and b>0 then if type((a^b+b^a)/n, integer)

then print(n); break; fi; fi; od; od; end: P(10^9);

MATHEMATICA

tst[n_]:=Catch@Block[{a, b}, Do[a=Floor[n/10^k]; b=Mod[n, 10^k]; If[Mod[ PowerMod[a, b, n] + PowerMod[b, a, n], n]==0, Throw@True], {k, IntegerLength[n]-1}]; False]; Select[Range@1000, tst] (* Giovanni Resta, May 12 2015 *)

CROSSREFS

Cf. A253824, A253825, A255725, A255726.

Sequence in context: A039480 A023688 A118157 * A261108 A147194 A147398

Adjacent sequences:  A257894 A257895 A257896 * A257898 A257899 A257900

KEYWORD

nonn,base

AUTHOR

Paolo P. Lava, May 12 2015

STATUS

approved

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Last modified July 9 18:08 EDT 2020. Contains 335545 sequences. (Running on oeis4.)