GoofyCoin question -- why pk, why not sk? At 45:43 in this video (from the Princeton Coursera Bitcoin and CryptoCurrency Course), they talk about GoofyCoin and show the image below.

My question is, why does it say "signed by pk"... isn't the actual signing done using the "sk", and the verification done using the "pk"?



Update: I think I figured out what's going on here, and am leaving this here for future reference. signed by pk(goofy) means "signed by the identity of goofy" because as mentioned in an earlier video, pk = identity
My question is, why does it say "signed by pk"... isn't the actual
  signing done using the "sk", and the verification done using the "pk"?


Yes. The actual signing is done by the "sk".

However, the signature is usually bundled with the public key which can be used to verify it. Thus it makes sense to more strongly associated the public key with the signature. Additionally it makes it trivial from a formal standpoint which public key to use for verification.

Furthermore, the mapping of public-key to private-key is 1:1 most of the time, making the choice of which key to use for indication here a matter of preference.