A068872 Composite numbers with all divisors congruent to 1 mod 10.

121, 341, 451, 671, 781, 961, 1111, 1271, 1331, 1441, 1661, 1681, 1891, 1991, 2101, 2201, 2321, 2501, 2651, 2761, 2911, 2981, 3091, 3131, 3421, 3641, 3721, 3751, 4061, 4141, 4331, 4411, 4631, 4681, 4741, 4961, 5041, 5071, 5371, 5401, 5611, 5731, 5921

Offset: 1

Author

Amarnath Murthy, Mar 19 2002

Keywords

• nonn

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A027750, A002808.

Programs

Haskell

a068872 n = a068872_list !! (n-1)
a068872_list = filter
   (all (== 1) . map (`mod` 10) . a027750_row) a002808_list
-- Reinhard Zumkeller, Mar 08 2012

Mathematica

fQ[n_] := ! PrimeQ@ n && Union@ Mod[ First /@ FactorInteger@ n, 10] == {1}; Rest@ Select[ Range@ 5950, fQ] (* Robert G. Wilson v, Mar 04 2012 *)

PARI

is(n)=!isprime(n) && n>1 && vecmax(factor(n)[,1]%10)==1 \\ Charles R Greathouse IV, Jun 19 2016

Extensions

More terms from Sascha Kurz, Mar 23 2002