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A244079 Numbers n with the property that it is possible to write the base 2 expansion of n as concat(a_2,b_2), with a_2>0 and b_2>0 such that, converting a_2 and b_2 to base 10 as a and b, we have sigma(a)*sigma(b) = n. 3
28, 120, 162, 196, 868, 1260, 1302, 1800, 2016, 2480, 3780, 4464, 6804, 7440, 8370, 9882, 22200, 32640, 34290, 35640, 40640, 73152, 127008, 187488, 213776, 489888, 572880, 602640, 674082, 1074528, 1077120, 1397088, 1536192, 1582560, 1662120, 1669164, 1781136, 1905120 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..38.

EXAMPLE

28 in base 2 is 11100. If we take 11100 = concat(11,100) then 11 and 100 converted to base 2 are 3 and 4. Finally sigma(3)*sigma(4) = 4 * 7 = 28;

120 in base 2 is 1111000. If we take 1111000 = concat(111,1000) then 111 and 1000 converted to base 10 are 7 and 8. Finally sigma(7)*sigma(8) = 8 * 15 = 120.

MAPLE

with(numtheory): P:=proc(q) local a, b, c, k, n;

for n from 1 to q do c:=convert(n, binary, decimal);

for k from 1 to ilog10(c) do

a:=convert(trunc(c/10^k), decimal, binary);

b:=convert((c mod 10^k), decimal, binary);

if a*b>0 then if sigma(a)*sigma(b)=n then print(n);

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

MATHEMATICA

f[n_] := Block[{d = IntegerDigits[n, 2], len = IntegerLength[n, 2], k}, ReplaceAll[Reap[Do[k = {FromDigits[Take[d, i], 2], FromDigits[Take[d, -(len - i)], 2]}; If[! MemberQ[k, 0], Sow@k], {i, 1, len - 1}]], {} -> {1}][[-1, 1]]]; Select[Range@ 100000, MemberQ[DivisorSigma[1, #1] DivisorSigma[1, #2] & @@@ f@ #, #] &] (* Michael De Vlieger, Jul 07 2015 *)

CROSSREFS

Cf. A000203, A258813, A258843, A259831, A259832.

Sequence in context: A251320 A233372 A220755 * A101095 A232403 A219851

Adjacent sequences:  A244076 A244077 A244078 * A244080 A244081 A244082

KEYWORD

nonn,base

AUTHOR

Paolo P. Lava, Jul 06 2015

STATUS

approved

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Last modified October 17 22:21 EDT 2019. Contains 328134 sequences. (Running on oeis4.)