A325175
Numbers k such that s(k) = s(k+1) = s(k+2) = s(k+3) where s(k) is the sum of divisors of k that are smaller than sqrt(k) (A070039).
Original entry on oeis.org
2, 1190499411, 7896840789, 9190223334, 10546199943, 13239147654, 16488284454, 19818736503, 19927041063, 20401914903, 20551875063, 24219563814, 25442242503, 27391724583, 34539825543, 35714513463, 37014168183, 44120613543, 44130940614, 44514172134, 48647900639
Offset: 1
2 is in the sequence since A070039(2) = A070039(3) = A070039(4) = A070039(5) = 1.
A334021
Numbers k such that s(k) = s(k+1) = s(k+2), where s(k) is the sum of unitary divisors of k that are smaller than sqrt(k) (A334019).
Original entry on oeis.org
2, 3, 7, 1420, 2505, 11860, 64060, 64485, 113413, 158020, 205365, 332658, 465272, 522764, 611085, 614538, 635053, 664033, 748484, 771138, 839213, 881565, 1011793, 1090788, 1190685, 1248645, 1306605, 1488088, 1607367, 1613190, 1836018, 1884914, 1911940, 2286913
Offset: 1
2 is a term since A334019(2) = A334019(3) = A334019(4) = 1.
-
s[n_] := DivisorSum[n, # &, #^2 < n && CoprimeQ[#, n/#] &]; seq={}; s1 = s[1]; s2 = s[2]; Do[s3 = s[n]; If[s1 == s2 && s2 == s3, AppendTo[seq, n - 2]]; s1 = s2; s2 = s3, {n, 3, 10^5}]; seq
Original entry on oeis.org
17784450, 28873348, 38990474, 44534923, 48780675, 85648274, 95438475, 100982924, 111100050, 157757850, 184508323, 188754075, 225621674, 240956324, 251073450, 308820148, 318937274, 334271924, 365595074, 371139523, 378806848, 391046850, 437704650, 505568474, 511112923
Offset: 1
-
f[p_, e_] := p^(2^(-1 + Position[Reverse@ IntegerDigits[e, 2], ?(# == 0 &)])); s[1] = 1; s[n] := s[n] = Times @@ (Flatten@ (f @@@ FactorInteger[n]) + 1);
Select[Range[10^8], (s1 = s[#]) > 1 && s1 == s[# + 1] == s[# + 2] &]
-
s(n) = {my(f = factor(n), k); prod(i = 1, #f~, k = apply(x -> 1 - x, binary(f[i, 2])); prod(j = 1, #k, if(k[j], f[i, 1]^(2^(#k-j)) + 1, 1)));}
lista(kmax) = {my(s1 = s(1), s2 = s(2), s3); for(k = 3, kmax, s3 = s(k); if(s1 > 1 && s1 == s2 && s2 == s3, print1(k - 2, ", ")); s1 = s2; s2 = s3);}
Showing 1-3 of 3 results.
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