A070858
Smallest prime == 1 mod L, where L = LCM of 1 to n.
Original entry on oeis.org
2, 3, 7, 13, 61, 61, 421, 2521, 2521, 2521, 55441, 55441, 4324321, 4324321, 4324321, 4324321, 85765681, 85765681, 232792561, 232792561, 232792561, 232792561, 10708457761, 10708457761, 26771144401, 26771144401, 401567166001, 401567166001, 18632716502401, 18632716502401
Offset: 1
-
A070858 := proc(n)
local l,p;
l := ilcm(seq(i,i=1..n)) ;
for p from 1 by l do
if isprime(p) then
return p;
end if;
end do:
end proc; # R. J. Mathar, Jun 25 2013
-
a[n_] := Module[{m = 1, lcm = LCM @@ Range[n]}, While[!PrimeQ[m], m += lcm]; m]; Array[a, 30] (* Amiram Eldar, Mar 15 2025 *)
-
a(n)=my(L=lcm(vector(n,i,i)),k=1);while(!ispseudoprime(k+=L),); k \\ Charles R Greathouse IV, Jun 25 2013
A070835
Numbers n such that phi(reverse(n)) = sigma(n).
Original entry on oeis.org
1, 168, 1141, 1451, 2139, 2737, 3938, 7597, 11831, 19668, 21415, 23649, 31894, 32383, 40649, 52687, 59677, 121023, 133661, 136831, 146055, 148503, 172095, 190035, 245998, 276058, 302005, 302503, 307705, 396635, 410389, 504557, 516439, 539327, 571577
Offset: 1
phi(reverse(168)) = phi(861) = 480 = sigma(168), so 168 is a term of the sequence.
A252255
Numbers n such that sigma(Rev(phi(n))) = phi(Rev(sigma(n))), where sigma is the sum of divisors and phi the Euler totient function.
Original entry on oeis.org
1, 14, 61, 966, 1428, 9174, 15642, 19934, 22155, 27075, 36650, 48731, 51095, 54184, 57902, 59711, 61039, 89276, 98645, 113080, 126850, 140283, 142149, 154670, 165822, 190908, 197705, 198712, 202096, 203107, 247268, 274368, 274716, 307836, 311925, 331037, 366740
Offset: 1
phi(14) = 6, Rev(6) = 6 and sigma(6) = 12;
sigma(14) = 24, Rev(24) = 42 and sigma(42) = 12.
-
with(numtheory): T:=proc(w) local x, y, z; x:=0; y:=w;
for z from 1 to ilog10(w)+1 do x:=10*x+(y mod 10); y:=trunc(y/10); od; x; end:
P:=proc(q) local a, b, k; global n; for n from 1 to q do
if sigma(T(phi(n)))=phi(T(sigma(n))) then print(n); fi; od; end: P(10^12);
-
Select[Range[400000],DivisorSigma[1,IntegerReverse[EulerPhi[#]]] == EulerPhi[ IntegerReverse[ DivisorSigma[ 1,#]]]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Apr 15 2017 *)
Showing 1-3 of 3 results.
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