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

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