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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
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
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 March 28 04:13 EDT 2024. Contains 371235 sequences. (Running on oeis4.)