A141197 - OEIS

A141197

a(n) = the number of divisors of n that are each one less than a power of a prime.

1, 2, 2, 3, 1, 4, 2, 4, 2, 3, 1, 6, 1, 3, 3, 5, 1, 5, 1, 4, 3, 3, 1, 8, 1, 3, 2, 5, 1, 7, 2, 5, 2, 2, 2, 8, 1, 2, 2, 6, 1, 6, 1, 4, 3, 3, 1, 10, 2, 3, 2, 5, 1, 5, 1, 6, 2, 3, 1, 10, 1, 3, 4, 5, 1, 6, 1, 3, 2, 5, 1, 11, 1, 2, 3, 3, 2, 6, 1, 8, 2, 3, 1, 9, 1, 2, 2, 6, 1, 8, 2, 4, 3, 2, 1, 11, 1, 3, 2, 5, 1, 5, 1

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OFFSET

1,2

COMMENTS

A067513(n) <= a(n) <= A000005(n). [From Reinhard Zumkeller, Oct 06 2008]

a(A185208(n)) = 1. - Reinhard Zumkeller, Nov 01 2012

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = sum (A010055(A027750(n,k)): k=1..A000005(n)). - Reinhard Zumkeller, Nov 01 2012

EXAMPLE

The divisors of 9 are 1,3,9. 1 is one less than 2, a power of a prime. 3 is one less than 4, a power of a prime. And 9 is one less than 10, not a power of a prime. There are therefore 2 such divisors that are each one less than a power of a prime. So a(9)=2.

MATHEMATICA

a[n_] := Select[Divisors[n], PrimeNu[# + 1] == 1 &] // Length; Table[a[n], {n, 1, 105}] (* Jean-François Alcover, Aug 17 2013 *)

Table[DivisorSum[n, 1 &, PrimePowerQ[# + 1] &], {n, 103}] (* Michael De Vlieger, Aug 29 2017 *)

PROG

(Haskell)

a141197 = sum . map (a010055 . (+ 1)) . a027750_row

-- Reinhard Zumkeller, Nov 01 2012

CROSSREFS

Cf. A141198.

Cf. A049073.

Sequence in context: A260439 A182471 A078378 * A035207 A324829 A294618

Adjacent sequences: A141194 A141195 A141196 * A141198 A141199 A141200