A346651 - OEIS

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A346651

a(n) is the number of divisors of A139245(n) ending with 2.

1, 2, 2, 2, 3, 2, 2, 3, 2, 2, 3, 3, 2, 4, 2, 2, 3, 2, 3, 4, 2, 2, 3, 2, 3, 5, 2, 3, 3, 2, 2, 4, 3, 2, 3, 4, 2, 4, 2, 3, 3, 2, 2, 5, 3, 2, 6, 2, 2, 4, 2, 3, 3, 3, 2, 4, 2, 4, 3, 3, 3, 5, 2, 2, 3, 2, 2, 7, 4, 2, 4, 2, 2, 4, 3, 3, 3, 2, 3, 6, 2, 3, 3, 4, 2, 4, 2, 2, 5, 2

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OFFSET

1,2

COMMENTS

a(n) is odd if and only if A139245(n) either is the square of a number ending with 2 or has a unitary prime divisor ending with 7.

The term 1 appears only for n = 1 in corresponding to A139245(1) = 4.

LINKS

Table of n, a(n) for n=1..90.

EXAMPLE

a(14) = 4 since there are 4 divisors of A139245(14) = 264 ending with 2: 2, 12, 22 and 132.

MATHEMATICA

a[n_]:=Length[Drop[Select[Divisors[20n-16], (Last[IntegerDigits[#]]==2&)]]]; Array[a, 90]

PROG

(PARI) a(n) = sumdiv(20*n-16, d, (d%10) == 2); \\ Michel Marcus, Jul 26 2021

CROSSREFS

Cf. A000005, A017293 (numbers ending with 2), A017294 (squares of numbers ending with 2), A030432, A056169, A139245 (product of two numbers ending with 2), A346388, A346389.

Sequence in context: A103507 A219252 A290839 * A085694 A258570 A257572

Adjacent sequences: A346648 A346649 A346650 * A346652 A346653 A346654

KEYWORD

nonn,base

AUTHOR

Stefano Spezia, Jul 26 2021

STATUS

approved