A069970 Numbers k such that sigma(reverse(k)) = sigma(reverse(k-1)) + sigma(reverse(k-2)).

3, 4, 352, 525, 532, 564, 572, 782, 3783, 5242, 5762, 5784, 7852, 7884, 31732, 38817, 41736, 46194, 52942, 57842, 61146, 63075, 67266, 68853, 95418, 196313, 403194, 424292, 436642, 444382, 493592, 521812, 521853, 521856, 523682, 527067, 527452, 541132, 543442

Offset

1,1

Links

Michael S. Branicky, Table of n, a(n) for n = 1..138 (all terms < 10^8)

Example

sigma(reverse(352)) = sigma(253) = 288 = 234 + 54 = sigma(153) + sigma(53) = sigma(reverse(352-1)) + sigma(reverse(352-2)).

Mathematica

rev[n_] := FromDigits[Reverse[IntegerDigits[n]]]; Select[Range[3, 10^5], DivisorSigma[1, rev[ # ]] == DivisorSigma[1, rev[ # - 1]] + DivisorSigma[1, rev[ # - 2]] &]

Prog

(Python)
from itertools import count, islice
from sympy import divisor_sigma as sigma
def agen(): # generator of terms
    dr2, dr1 = 1, 3
    for k in count(3):
        dr = sigma(int(str(k)[::-1]))
        if dr == dr1 + dr2:
            yield k
        dr2, dr1 = dr1, dr
print(list(islice(agen(), 25))) # Michael S. Branicky, May 29 2024

Crossrefs

Cf. A000203, A004086.

Sequence in context: A348280 A304150 A305505 * A305169 A316759 A334471

Adjacent sequences: A069967 A069968 A069969 * A069971 A069972 A069973

Keyword

base,nonn

Author

Joseph L. Pe, Apr 29 2002

Extensions

More terms from Sean A. Irvine, May 28 2024

Status

approved