# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a087893 Showing 1-1 of 1 %I A087893 #18 May 24 2024 03:28:47 %S A087893 0,0,0,0,0,2,0,0,0,2,0,2,0,2,2,0,0,2,0,2,2,2,0,2,0,2,0,2,0,6,0,0,2,2, %T A087893 2,2,0,2,2,2,0,6,0,2,2,2,0,2,0,2,2,2,0,2,2,2,2,2,0,6,0,2,2,0,2,6,0,2, %U A087893 2,6,0,2,0,2,2,2,2,6,0,2,0,2,0,6,2,2,2,2,0,6,2,2,2,2,2,2,0,2,2,2,0,6,0,2,6 %N A087893 Number of numbers m satisfying 1 < m < n such that m^2 == m (mod n). %C A087893 The number of nontrivial unitary divisors of n (i.e., excluding 1 and n). - _Amiram Eldar_, May 29 2020 %C A087893 a(n) first deviates from b(n) = 2*A079275(n) at a(210) = 14 <> b(210) = 12. - _Georg Fischer_, May 23 2024 %D A087893 C. R. J. Singleton, "Prime Function Problem": Solution to Problem 2355, Journal of Recreational Mathematics, Vol. 29(3) pp. 232-234, 1998. %H A087893 G. C. Greubel, Table of n, a(n) for n = 1..5000 %F A087893 a(n) = 2^omega(n) - 2 (for n > 1). %t A087893 Join[{0}, Table[2^(PrimeNu[n]) - 2, {n, 2, 50}]] (* or *) Table[2*Module[{c = PrimeNu[n]}, (c (c - 1))/2], {n, 1, 20}] (* _G. C. Greubel_, May 20 2017 *) %o A087893 (PARI) concat([0], for(n=2, 50, print1( 2^(omega(n)) - 2, ", "))) \\ _G. C. Greubel_, May 20 2017 %Y A087893 Cf. A001221, A007875, A034444, A079275, A309307. %K A087893 nonn %O A087893 1,6 %A A087893 _Lekraj Beedassy_, Oct 13 2003 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE