A338917 - OEIS

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A338917

a(n) = sum_of_digits(a(n-1)^a(n-2)) where a(1)=1 and a(2)=2.

1, 2, 2, 4, 7, 7, 25, 34, 151, 331, 1690, 3265, 26449, 64528, 574513, 1671208, 16090657, 54199564, 559922497, 2133503863, 23506132363

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Table of n, a(n) for n=1..21.

GeeksforGeeks, Sum of digits of a given number to a given power

FORMULA

a(n) == 7 (mod 9) for n >= 5. - Hugo Pfoertner, Nov 15 2020

EXAMPLE

for n=6, a(6) = sum_of_digits(7^7) = sum_of_digits(823543) = 25

MATHEMATICA

a[1]:=1; a[2]:=2; a[n_]:=Total[IntegerDigits[a[n-1]^a[n-2]]]; Array[a, 19] (* Stefano Spezia, Nov 15 2020 *)

PROG

(SageMath)

a, b=1, 2

L=[a, b]

for n in [1..17]:

c=b^a

c=sum(c.digits())

L.append(c)

a, b=b, c

print(L)

(PARI) a338917(nmax)={my(x=vector(nmax)); x[1]=1; x[2]=2; for(k=3, nmax, x[k]=sumdigits(x[k-1]^x[k-2])); x};

a338917(18) \\ Hugo Pfoertner, Nov 15 2020

CROSSREFS

Cf. A007953.

Sequence in context: A244011 A065968 A105669 * A256963 A355306 A019657

Adjacent sequences: A338914 A338915 A338916 * A338918 A338919 A338920

KEYWORD

nonn,base,more

AUTHOR

Sean Lestrange, Nov 15 2020

EXTENSIONS

a(20) from Hugo Pfoertner, Nov 15 2020

a(21) from Chai Wah Wu, Nov 19 2020

STATUS

approved