A177929 - OEIS

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A177929

a(1)=4. a(n+1) = a(n)+d-1, where d is the smallest prime divisor of (a(n)-1)*(a(n)+1).

4, 6, 10, 12, 22, 24, 28, 30, 58, 60, 118, 120, 126, 130, 132, 138, 274, 276, 280, 282, 562, 564, 568, 570, 1138, 1140, 1146, 1150, 1152, 2302, 2304, 2308, 2310, 4618, 4620, 4650, 9298, 9300, 9316, 9318, 9324, 9328, 9330, 9336, 9340, 9342, 18682, 18684

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..48.

FORMULA

a(n) = a(n-1) + A020639(a(n-1)^2 - 1) - 1. - R. J. Mathar, May 31 2010

MAPLE

A020639 := proc(n) numtheory[factorset](n) ; min(op(%)) ; end proc:

A177929 := proc(n) local d1, d2; if n = 1 then 4; else d1 := A020639(procname(n-1)-1) ; d2 := A020639(procname(n-1)+1) ; procname(n-1)+min(d1, d2) -1; end if; end proc:

seq(A177929(n), n=1..52) ; # R. J. Mathar, May 31 2010

MATHEMATICA

lpf[n_] := FactorInteger[n][[1, 1]];

a[n_] := a[n] = If[n == 1, 4, a[n-1]+lpf[a[n-1]^2-1]-1];

Table[a[n], {n, 1, 50}] (* Jean-François Alcover, May 13 2023 *)

CROSSREFS

Cf. A020639.

Sequence in context: A272777 A310581 A310582 * A086493 A234965 A320881

Adjacent sequences: A177926 A177927 A177928 * A177930 A177931 A177932

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, May 15 2010

STATUS

approved