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A086470 Numbers k such that psigma(k) = psigma(k+1), where psigma(k) = A086469(k). 2
9, 21, 33, 44, 57, 93, 141, 169, 177, 201, 213, 258, 381, 393, 426, 453, 501, 537, 633, 670, 678, 717, 762, 921, 933, 1041, 1137, 1266, 1293, 1317, 1401, 1437, 1590, 1641, 1686, 1713, 1761, 1821, 1857, 1893, 1941, 1990, 2181, 2217, 2361, 2433, 2509, 2517 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n =3p and n+1 = 2q where p and q are primes then n is a member.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

9 is a member as psigma(9) = 1+3 +9 = psigma(10) = 1+2 +10 = 13.

MATHEMATICA

a[n_] := Module[{d = Rest[Divisors[n]]}, 1 + Total@DeleteDuplicatesBy[{#, Sort[FactorInteger[#][[;; , 2]]]} & /@ d, Last][[;; , 1]]]; s={}; a1=0; Do[a2 = a[n]; If[a1 == a2, Append|To[s, n-1]], {n, 1, 2500}]; s (* Amiram Eldar, Jul 20 2019 *)

CROSSREFS

Cf. A086469.

Sequence in context: A243703 A133929 A325573 * A176256 A017629 A216240

Adjacent sequences:  A086467 A086468 A086469 * A086471 A086472 A086473

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Jul 21 2003

EXTENSIONS

Corrected and extended by David Wasserman, Mar 07 2005

Offset corrected by Amiram Eldar, Jul 20 2019

STATUS

approved

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Last modified June 5 09:46 EDT 2020. Contains 334829 sequences. (Running on oeis4.)